Is he any good? I'm pretty new to dominion, but I'd like to think I'm a pretty good Ezreal. I'm good at playing kogmaw in 5s, but I'd like to know if and how viable he is in dominion.

Short answer, yes. If you're good at playing him in SR, you'll understand positioning well enough to do well. However, there are far more gap closers in dominion, so be prepared to die more. If you're playing dominion at a level where you can't rely on your teammates, then koggie can be useless, since he needs a decent front line to function. Also note that you often won't be able to build glass. A Frozen Mallet can do wonders.

As an added bonus, if your team is having trouble with a tough bot lane (Urgot, Yorick, etc.), then Kog can be sent bot late game to eat them. I DON'T recommend sending kog bot from the start, since he's very vulnerable to ganks.

On a coordinated team he's decent. But if you already play Ez just keep doing that cause Ez is much better overall.

I love Kog'Maw for his short cooldown long range poke and crazy damage. You do need a team that can help keep people off of you. If your front line consists of Nidalee, Irelia and Warwick it's going to be tough. I find I can win against most bruisers 1 v 1 very early on with him though which is really nice when you're playing an AD carry.

Alright well thanks for the help guys. What I gather from this is:
Kogmw is good with coordinated teams, so I probably should not use him in solo queue at the elo I'm at, where people still believe risking everything for an extra point when we already have three and running ghost just for the windmill race is good.
And that I should stick with Ez

He's good artillery that needs a little protection, and worse case scenario his passive is going to see a lot of use as dying on Dom is common.

I used to only play Koggles in Dominion, and when Jabebot let us check our elo for the first time I was around 1900 in Dominion.
(at my highest I was a bit over 2.2k)

He just scales so ridiculously hard, can repeatedly interrupt caps from an incredibly long distance, and getting kills with the passive is great fun.

If you are an AD carry player, I highly recommend picking up Kog'maw.

Bleh!