I DM exclusively on Roll20, so having this on my second monitor will be a game changer.  Having all of my books at my computer desk is a pain and cumbersome.  

I've only poked around on mobile as I'm at work today, so my only comment so far is that the forums don't work properly on my iPhone.  When vertical, the page is blank outside of the header and the post box at the bottom.  I have to turn my phone 90 degrees to read the posts.  

First thoughts 

I got into the phase one beta. It is fine. You have to use Twitch account to log in, which is annoying but no more than any other "you must use this other sites credentials" requirement.

The phase one is not terribly impressive because they haven't added any of the hard bits. Right now it is just the stuff you can find on any SRD page out there. Less actually. There is neither a class nor a race breakdown in their compendium, so you actually get more out of the fan made SRD pages in that. I guess they are keeping those in the character creator. There are hints you can unlock books not part of the SRD down the road, but no details yet. The site navigation is relatively easy. The page collapsed on me a couple of times but quickly recovered.

Right now it is just okay, but not a whole lot there to sell it beyond my usual srd sites. Once we get into phase one or two, that should change.

First thought is that it looks pretty good. I very quickly wanted to see a search feature in the rules compendium (if it is there, I didn't see it). It would also be useful if game terminology was hotlinked (both in rules pages that refer to other rules and in the magic item, monster entries, etc)

First thoughts, hmm where to start, the site is Clean and easy to Navigate, the infomation is well presented and again easy to find, hopefully when the next phase comes out it will be as clean and easy as this

I wasn't ready for how unpleasing to the eye the compendium is :/

I'm quite happy that Wizards approached this endeavor and I know they will continue to improve it. 

Thus far I've had the following technical issues (running on Safari mobile):

- writing and submitting to the forums; text often takes a while to show up and is a bit buggy

- drop down menus bring up my keyboard which interferes with selection (choosing my time zone for example)

- the page doesn't adjust automatically to find my text cursor and make it visible

- when loading a forum page, 25% of the time the posts will be all smashed together at the top and non-visible

So far so good!  The last time my buddies and I played we had two questions. One was regarding a standard healing potion.  The other question revolved around magic missle. I was able to find the answer to both questions in about two minutes tonight!  Impressive!

I think so far they have done something right. This is going to be a great tool for those DM's who use laptops and don't like to have a bunch of physical books around. The UI is simple, but thats a good thing in this case. Everything is easy to find and navigate through. I enjoy it so far, look forward to more features and data!

Loving everything about this. I only wish that the monster search by type didn't toggle. Maybe there could be a check box beneath each one you want to display, but then clicking on each type shows only that type? Or maybe just the check boxes. That would probably work a little bit more intuitively.

First thoughts:

I love the search function and filters. Very nice and very well executed IMO.

I love that there is artwork! I don't know why, but I expected this to be more utilitarian than what it is.

To be quite honest, it has exceeded any expectations that I had before going in. I feel like I could hours just playing around with this. Reading rules stuff, reading about carious items, becoming more familiar with monsters that I may have skimmed over in the MM, etc.

Call me a happy camper, thus far. :o)

Oh, and also... the forums need color. Something kind of oppressive about the black and white. Maybe even just some shading?

Not the biggest fan of the Twitch thing but that doesn't bother me. I can't wait for this to be open for all of the MM, SCAG, and Volo's stuff.

My thoughts:

Having every iteration of +N weapons in the magic weapon tag is...crowded.

No class or race stuff in the SRD is problematic, and the lack of cross linking, as mentioned before, is a let down. I personally dislike running spellcasting creatures because I always have to go look up a spell in the middle of combat, bogging things down and halting momentum and suspense. Beyond could easily both let me either click through to spells on casters or have a pop up.

All of these things said, this is literally day one of the release, so don't be too hard, things will get get better. I can't wait for phases 2 and 3, and for an app so I can use this on my ipad, which is what I run off of with Google Docs when I DM and a PDF reader for my sheets when I play.

The look and feel of the site is 9/10 imo. One thing I'm not particularly fond of is the harmonica menus for items, spells and monsters, and the way the descriptions are. I don't know if that makes any sense, but it's too long horizontally, and too short vertically. Makes reading odd, seeing as all other D&D material is pretty much always in two columns, and monsters, even homebrew, usually have a specific table styling. I'd not want to see the exact same style, but the same format.

I like how things are broken up category wise and everything loads quickly, so yay.

Things I don't like so far:

Adventuring Gear, Weapons, and Armor layouts are very annoying and almost unusable. It needs to be more table formatted (closer to monster entry maybe if the formatting can't be fixed).

i really don't like the colored ability scores (or the icon really, but less so) in the monster stat blocks. They serve no real purpose and it feels like you guys just wanted to add color for some reason.

Multiple entries for things like Ademantite armor under Magic Items. There has got to be a better way of doing this. It's ugly and cluttered.

For spell list, the Spell level should be as selectable as the class (like a second row beneath the class ). Putting it in advanced filters makes me have to:

1. press advanced filter,

2. hit the lv bar,

3. select the lv (if multiple Lvs repeat 2&3)

4. Select search (since the list doesn't auto update)

5. Press enter.

Past these annoyances that can easily be fixed. It works well enough so far. I'll give more feedback as I try it further 

 I really like the layout and UI with one exception. The compendium section doesn't have a search function that I can find. This would be a useful feature, especially when you need to relearn how grappling works ASAP.

So far I have only used dndbeyond on my mobile device and it has been running very well. I will be trying it more as a player and a DM and I will be posting my experiences as they unfold.

So far the features seem to be better than the one WotC tried to put out.

By which I mean this one.

Quote from Lorathorn >>

So far the features seem to be better than the one WotC tried to put out.

By which I mean this one.

There are benefits to the other one that you linked to. For example, if I want to look for a specific ruling in the SRD, I can still search the page by pressing ctrl+f to search for something. But yes, I mostly agree that the look and feel of D&D Beyond is better.

Oh, something to add to the monster list, I think, is a tag for monsters with spellcasting of one sort or another. Or have the ability to add racial things to NPC stat blocks, so with a click or three I can have a Kenku Assassin, etc.