Well, I'll jump in. I started using MT way back in the day (around 2002). I stayed with it, despite the fact that IMHO it was a royal PITA to tweak your templates or design for it b/c of the myriad templates involved for everything. Index, archive, three kinds of archive templates, comment templates, etc., etc., etc. I got real clever at using "modules" to stash pieces of code.
When the licensing debacle in Spring 2004 was in full swing, I started using WP. Even back then (WP 1.2) I thought WP had it all over MT, simply because of the fact that you didn't need all those other templates and files. And .. because you didn't need to sit there while your blog with three or four years' worth of posts "rebuilt" itself. Of course, MT now has dynamic PHP capabilities (MT has taken more from WP than WP has--or needs to--from MT, and there's a reason for that).
And rather than employing universally understood PHP code, MT uses its tag system which can take some getting used to. Back then, the MT Forum (not run by any MT staffers, but by dedicated MT fans and users) was active and extremely helpful. That community, as I knew it, unfortunately does not exist anymore.
Now all this, obviously, was back before it became Open Source so maybe this will be the kick in the pants it needs. To be honest with you, I'm not so sure. Of course it has its loyal fan base that will stick with it no matter what.
I downloaded MT4 when it came out a few months ago and played around with it, but it's not something I see spending a lot of time on, and I design web sites for a living. The fact of the matter is quite a good bit of my business comes from my former MT clients wanting to move to WP and have their designs ported or new designs. I've done ONE MT design in the last 18 months. And that was for someone who had paid for a commercial license and had a vested interest in sticking with the chosen blog app. FWIW.