I'm also running a hacked OS. Samsung Moment on Sprint. First it was supposed to be updated in late March, and since then it has been "any day now" for over a month.
I think there's a lot of passing the buck between the carrier and the manufacturer. I don't know whether to blame Samsung, HTC, or Sprint for the delays. I've heard unconfirmed rumors that Sprint wants to update both the Hero and the Moment at the same time, so it's possible that the update is finished for one but not for the other. And I still have no clue what's taking them so long. It's not like 2.1 doesn't exist or something. It's been around for months.
So yeah, a couple days ago I got tired of waiting and installed 2.1 on my Moment. Started with that, then installed a custom ROM, then tried a newer release (DD10 instead of DD03) with another custom ROM. So far the biggest advantages have been the slide to unlock and slide to answer features along with compatibility with more apps. There were a lot of apps I wanted to use but couldn't because they were only for newer versions of Android. Examples:
Helix Launcher - Pretty simple replacement home. Unlike most home replacements, it's not overburdened with features so it's about as fast as the stock home. Lets you add four extra apps that are available on all home screens. Also lets you add extra home screens (I don't really need more than the stock 3 but you can have more if you want).
Google Navigation - Much improved over Maps. Google Nav gives you turn-by-turn directions without having to use a shitty Sprint/Verizon/whatever navigation app.
Google Goggles - Not that useful, but neat. Take a picture of just about anything and it will analyze it and give you more information. Converts pictures of business cards into contact information, pictures of businesses into reviews of said businesses, and pictures of landmarks into travel information.