I have tried almost every rom I could find, I have to say there are three that stood out, 1st is Moonscapex's "1.6"(fast, light on mem, and a beautiful black theme), 2nd is "Very Lt. Sense" from Dev O.a.T.(I liked this one because it includes Max Sense, I think its the only rom for the Diamond that does, but it uses up a bit more mem), the 3rd is "Energy Rom" from Dev NRGZ28( my personal favorite, it is good on mem, has in my opinion a beautiful theme, and is highly customizable). So those are just my favorites, I still use my Sprint Hero as a daily phone, so I have had an opportunity to flash a lot of roms and add-ons to my Diamond and I can honestly say there are bugs in quite a few roms but the three I suggested are the most stable I have used.
Now for the fun stuff, I am an early adopter of the android platform, so I am of course going to suggest XDAndroid, this is not flashable, I mean it will not replace WM, think of it as Parallels(a virtual machine), it is fairly simple to add to your phone, it is not the most functional at this point but the Devs involved are working very hard, and it is working well enough at this point that you can run it and see if you like Android(I know you will
), and I think its great that we have a choice of 1.6/2.1/or 2.2!!
Also, XDA is great, really my favorite community, so it embarasses me to say this, but for roms for our CDMA Diamonds PPCGeeks.com is the place to go.
Ok, well let me know if there is anything else
Moonscapex's "1.6" - http://forum.ppcgeeks.com/showthread.php?t=114974
O.a.T.'s "Very Lt. Sense" - http://forum.ppcgeeks.com/showthread.php?t=127266
NRGZ28's "Energy Rom" - http://forum.ppcgeeks.com/showthread.php?t=74084
XDAndroid - http://forum.xda-developers.com/showthread.php?t=701566