It's funny, sounds like a lot of us are having similar experiences.
I had Handcent since the first day I got my droid x, 5 months ago or so. I text a lot. I customized the hell out of it, and LOVED it. I really thought it was genius. For Christmas, my dad got a Droid X, and I happened to be in town so I helped him set it up (he couldn't even answer a call... seriously).
So I thought to myself "hm, maybe I should take a couple risks. Instead of my default, Miren browser, I'm going to try this new dolphin mini for his phone (he'll really never know what he's missing either way, so it's low risk). I liked it, he did too. He said he wanted text pop ups. I thought handcent right away. Then I remembered reading GO SMS on xda forums. I remember downloading it a few months ago and it was too early in it's creation and really weak all around, but I tried it on his phone anyway to test the updates. I couldn't tell what it was that I liked so much about it. Just that it seemed the same on the surface as handcent, at about a third of the size. So I was intrigued.
I popped it on my phone, started playing with the customization, and within an hour I had removed handcent. The themed pop ups on go are completely awesome. But more importantly, it really is noticeably faster. Handcent has a pretty big upside when it comes to mms (you can now use their server to send big pictures instead of resizing them). BUT, GO SMS has speed, flash, usability, and is moving at an incredibly rapid pace. I reported a bug and it was fixed THAT night.
The devs behind GO are very talented, and really represent what Android is all about: New, fresh talent. I'm a huge fan and recommend everyone try it out for a day.
It makes me want to try GO Launcher, as a matter of fact... But I'm pretty attached to Fede...