Comparing the Fathom to the TP2 is like night and day. My mother has a TP2 and I've always felt impatient when trying to use it after I've used my Fathom.
The Fathom's only shortcoming is the limited amount of RAM, but when configured right, you'll never notice. I usually have 60+MB of free memory on my phone and never go under 30M while using it. I've been able to run Skype, Google Maps, my email, Slacker, and emulate a PS1 game all at the same time.
I use Titanium on my phone, and a few of the panels that I have eat a decent amount of RAM. when i switch to Today I usually have 90+ mb free on a fresh boot.
There are a few hiccups right now with the Fathom that I haven't figured out. The current ROMS that are built still have an issue with 3G connectivity that makes the 3G drop if you're on Verizon and run some heavy data for a few minutes (Torrent, Skype, Slacker, Downloads) But otherwise it's fine.
If you can find an extended battery and battery door (about $30-40 online) then you'll really see the phone shine. On a GSM network, the Fathom will get about 2 days without data usage, and my Verizon one will get me about 26 hours on a charge when I remember to close out programs.
It seems to be a stable phone for the most part. When I get some more time (I have Thanksgiving break soon, and I plan to sync my Kitchen to my netbook) I'll see if I can build another ROM or two that are more lean and less bug filled.
For now, there's 4 ROMs posted on PPCGeeks. The ones without PowerMobia are somewhat broken, but easily fixed (just copy\paste a dll file into \windows and reboot)
There is zero lag on the phone when running programs. Even Opera Mobile 10 rarely shows a loading screen. Most programs don't even get that far. They're up and running as soon as you hit the icon.
If there's any requests that you have for Roms, direct them to the thread on PPCGeeks and we'll take care of the rest.