Should I upgrade from 10.7.4 to ML?

Greetings all,

I'm running an iMac 2.5HGz w/4GB of ram. I bought the machine to port my game to the iOS, however it's grown on me so much that I pretty much use it more then my PC now except when I'm coding. It's a great computer!

I bought the iMac in early Jun and it came w/OS X v10.7.4.

I see now that I can get Mountain Lion, 10.8 for $20, which seems like a no brainer, except I'm really sort of wondering if it's a useful upgrade and if it will introduce any aggravation. Honestly, things run at a pretty good clip on this computer, I have as much integration w/my iOS devices as I need and I'm thinking that maybe I'll just skip it unless there is some compelling reason not to.

So what is compelling about ML that would make it worthwhile? Will my iMac run as fast, faster or slower w/ML?

Thanks!

Wag