Plenty of benefits, number one in my mind being the ability to upgrade to the latest version of Android MUCH sooner than waiting for your manufacturer to release an upgrade.
The only real con is you'll void your warranty. If that's not a concern for you then I say go for it.
Just as a side note, I have not rooted my Droid yet because of the warranty issue. My workplace is ridiculously hard on phones and I'm positive this one won't make it past the warranty expiration date alive...
I've only bothered to root and throw a custom rom on a couple of phones. I did it to a Sony Xperia X10 so that I could sell it easier and with a Samsung Captivate. I got tired of waiting for official update and hate their Touchwiz interface so I loaded it with the Cognition rom. Cyanogenmod is generally gonna be the best with HTC phones. Every time it adds functionality and speed.
I'm mainly using an HTC Inspire and may end up using Cyanogen with that if I decide to keep it.
I have only overclocked a Palm Pre Plus because it seriously needed it. Most HTC phones are already so quick I don't see the need for it.
MT4G is a sweet phone, btw.