1) What brand/model did you buy, how long have you had it, and would you buy again?
I bought an M2. I'm glad I did because printing 2 shirts at once or 4 pockets at once is HUGE!
2) Are you happy with supply costs? Is there non-manufacturer ink available, or only mfg supplied?
Col-Desi overcharges for ALL their supplies, not sure why. The machine is awesome and their support is fantastic, but for supplies I would go elsewhere. Heat Press Inc sells the ink much cheaper (it's actually the same ink, Dupont, just relabeled). Image Armor is awesome Pre Treat for almost half the price of DTG Brand Pre treat.
3) Do you have a pretreatment "device" or use a sprayer?
We just order a Speed Treater TX, but right now we use a Wagner. If you know what you are doing, pretreating isn't bad but is a HUGE learning curve. Getting a machine will allow you to hire any worker at min wage to pre treat shirts, and they will all come out the same. Beware of older pre treat machines though, they tend to break down a lot. The new ones that have come out seem to be a lot better designed.
4) If you were buying today, what would you buy?
I would buy another M2 or an M4. But honestly my next step is an auto screen press.
5) Getting started, what was unexpected in the learning curve?
Honestly, I don't think there is that big of a learning curve to DTG. I know everyone says that, but I find it is over stated. Now, if you are the type of person that has trouble checking your email or hates computers, then yeah it's going to suck big time! I myself was able to print several perfect shirts my first day. I had a few mess ups, but the big learning curves are when you have to figure out whats wrong, like un-level platens/RGB Color correction/raised areas from pockets/scorching white shirts/etc.
I have a background in engineering and software, so maybe that helped me, but if you are the type of person that gets frustrated easily and "just wants it to work" then DTG is going to drive you nuts just like ANY other form of printing. But if you are the "make it happen" type of person, then it will be like just another day at the office.