It is not very popular because it has always been an overpriced phone, it is selling for +250 used and was like $180 + 2 years of your life.
I don't think that phone warranted such a price because of the bump in the ram and the keyboard, Tmobile just butchered android in that phone with their "magic key" which is just the google search with free bloatware included i don't know if MT3 Slide will ever get 2.2 but it seems like a big no unless of course you root and download custom roms.
I think is a nice phone but i've had the original MT3G and the screen was too small to type vertically, you had to tilt your phone 90 degrees but considering the added bulk and the $100 keyboard, i'd rather have the original MT