I hear a lot of people say that this version of the Horn of Blasting is kinda bad. 20% to blow up on any use, including the very first one, is a gamble not many people want to take.

It was even worse for AL, back when a non-consumable magic item that blew up due to being used outside of story reasons still counted towards your magic item total. 

If I hand this out as a DM, I'd make the first use of each day not run the 20% risk.

If you roll within that 20%, does it blow instead of dealing thunder damage or after it deals thunder damage?

Yeah

Quote from Hippopotamouse >>

Yeah

 I just wanted to contribute that this is a quality contribution to the discussion.

It says each time the horn is used there is a 20% chance it causes it to blow up. So it had to be used first. 

Sounds reasonable to me!

My party's wizard just used it for the first time..... and then rolled 20/100. Dealt 3× as much damage to herself as she did to the flameskull, and STILL didn't kill it. 

I agree with the concerns about the value of this item. I would modify it as follows:

1. No risk of damage for the first use that day.
2. Every additional use that day adds a 20% risk of causing the horn to explode after it is used.
3. DC15 Dexterity saving throw, 10d6 fire damage on a failed saving throw, 5d6 on a successful one
4. Fire damage is in a 10' radius.

The result is as follows:

• First use - 10d6/5d6 Thunder damage, no risk of explosion
• Second use - 10d6/5d6 Thunder damage, THEN 20% risk of 10d6 fire damage
• Third use - 10d6/5d6 Thunder damage, THEN 40% risk of 10d6 fire damage
• Fourth use - ... 60% risk
• Fifth use - ... 80% risk
• Sixth use - ... Guaranteed

You could even add some flavor around stress cracks appearing (perhaps using mending to fix). It might also retain the percentage (like levels of Exhaustion), taking a full day of non-use to reduce the percentage by 20%. So if used 3 times one day (so a 40% risk), it retains that 40% risk the next day, but if not used that day, the following day it is down to 20%, and the day after (without use) it is back to 0%.

Note that this creates fascinating trade-offs at multiple levels for the player and the party. That makes it really interesting - useful to an extent, and useable with great risk thereafter.

I found one of these and used it to see what it does, and it blew up first time lol

“Creatures and objects made of glass or crystal have disadvantage on the saving throw and take 10d6 thunder damage instead of 5d6.”

Correct me if I’m wrong but the only creatures I know that are made of crystal are gem dragons.

Also, Artificers can make this thing for free…

2 person party, Artillerist and Forge Domain Cleric. Every long rest the Artificer pumps out several of these and hands them to the cleric, who has Fire Immunity.

Kaboom

What about having the imp familiar use it? They have fire immunity and they can carry 45lbs, and they have a mouth so they can blow the horn,

I really prefer the original version, the version you point out makes the item more disposable.