So I currently have a Rooted X and I may have something lined up where I might trade for a Fascinate. I see the root thread here is kinda discombobulated, no stickies or root guides all bundled up nice and neat like in other threads (BOOO D13, Frisco and OTD you guys are slippin'

) What Roms do we have, OC?, Themes, etc.... I'm up at 330 am with my 2 year old daughter and its hurting my brain trying to gather all the information I'm looking for.
Yeah a sticky or two here would be nice

. XDA hosts pretty much everything that is available for the fascinate at the moment, but when I find af in general to be a friendlier place to go with general questions and when seeking help. Also, this may sound dumb, but the color scheme on xda for some reason bothers me and makes me not want to spend as much time there as I do.
Anyway, back to your question. Selling my DX and switching to the fascinate has been great for me. In the way of themes there are currently less than the DX but that is more a product of the DX being out an extra 2 months than the SF, there are several from modified nextheme, to the black sense bar, incognito, etc and they are over at xda.
The roms that are available are 2.1 based as we are all awaiting 2.2, but unlike the dx with how amazingly fast the sf is, minus flash, you don't need it as badly. There are a few over at XDA, personally I'm happy with my current set up so I'm waiting for froyo before I grab a rom.
Overclocking is where the fascinate shines at the moment. The voodoo lag fix for galaxy s phone has been ported over now, and corresponding kernels with it are available. Again, over at xda there are low voltage and standard voltage kernels available from 1000 mhz to 1250. Some peoples phones work great with low voltage (mine doesn't). Voodoo really is the single biggest thing out imo for the fascinate at the moment. It makes the phone fly. That being said, read over everything very carefully as voodoo and backups and restore are slightly different than other android phones because of the way it rewrites your system. If you have issues and need to restore your phone, disable it first. Myself (and many others) didnt and had a fun few hours fixing it.
There are a lot of knowledgeable and helpful people here that I'm sure can give you much better answers than me, but if you make the swtich you won't regret it!