how good is this item?

So good for hiding, spying on important conversations, and escaping!

Useful for most, probably amazing on a Simic Hybrid with Manta Glide! Whenever you're in dim light, you have a free eighty feet of forwards movement! Nice!

Flavour Idea: If you're an Artificer with this infusion, it's probably not unreasonable to ask your DM if one could be created with a specific other CR0 flying beast in mind. The Owl and Giant Fly forms both seem appropriate, as they both have Darkvision. The former would be a good getaway (and a really pretty cloak), while the latter is strictly for utility (while being potentially flavoured as the cloak weaving the terrifying form around you, which is way cooler than just being a magic spell). The Giant Fly can take a hit better, but it can't take any actions, removes your class features and still relies on concentration so it's not really breaking balance.

My half-eft Hexblade/Bard (9/3 lvl) must acquired a Cloak of the Bat. One of his traveling companions happens to have a Sun Sword. 

Pretty basic -- what happens if you are accidentally or intentionally exposed to light more bright than dim? If you're in mid-flight, does this create a crash hazard?

Quote from Iron_Theurge >>

My half-eft Hexblade/Bard (9/3 lvl) must acquired a Cloak of the Bat. One of his traveling companions happens to have a Sun Sword. 

Pretty basic -- what happens if you are accidentally or intentionally exposed to light more bright than dim? If you're in mid-flight, does this create a crash hazard?

For the purposes of the Fly speed, I would say yes, this absolutely is a crash hazard. Though, I would allow a falling character to attempt to dip out of the light (assuming it makes sense) in order to regain the fly speed. Would involve a DEX check of some sort. Thankfully characters who want this item, tend to have high DEX anyway.

For the Polymorph, though it wasn't asked and should be fairly obvious, being exposed to the light after poly-morphing would have no  effect

If exposed to light with no escape, do you land, or take damage from falling/crashing?

My Artificer reaches level 14

" I AM the BATMAN!!"

If you're in the air, you just fall

According to Xanathar's Guide, it would take an artificer proficient with weaver's tools (or leatherworking if you can convince the GM) 10 workweeks and 2000 gold to craft this item.  

You'd also need some rare resources from a specific creature with a CR of 9-12 (probably from a vampire ie blood, hair or teeth).  

You can cut that time and gold down to about 2 workweeks and 1000 gold if your artificer is over lvl 10.

Although it is also a 14th level Artificer Infusion 

Quote from KirisinIdril >>

Although it is also a 14th level Artificer Infusion 

 Yeah, but it's always better to get an item now, when you may need it rather than wait, and I rather have 2 of an item that's really helpful than only 1 especially one that helps with stealth.  

How many players are going to shout “Baaat”! Just before they polymorph? #whatwedointheshadows

For the vampire who is either overly dramatic or forgot that he/she can turn into a bat.

Perfect for the Dhampir Bloodhunter whose lycan form is a terrifying bat creature…

I am kinda annoyed that I cant use this in aerial combat since I need two hands to fly but still cool magic item for a spy.

could you create a version that have the back protected ac

What is that picture

Cast "Darkness" on something you're wearing, grab your cloak, fly away.

isnt this extremely busted for a thri-kreen since u have 4 hands?