It's going to be one year with my mP5 in a couple of weeks and I thought it would be a good idea to share what I found as it may help others.
First, DTG is amazing! I don't regret for a minute having entered this market. The learning curve is quite steep but when you start to master it, it is incredibly rewarding.
The mP5i is an interesting machine; I LOVE printing on white/light colors as I find it has great quality and it's fast. I am doing more and more jobs on it and I am even considering adding a night shift to make it run even more (my turnaround time is a week and a half atm with it running 8 hours a day).
Unfortunately I had to give up on dark colors; we don't do one-offs so those incredible profit margins are out of the way. We do actual production! 20/50/100 shirt runs (in a poor-ish country), which makes our profit margins be lower than what I usually see here.
Dark colors always gave us a lot of problems; we would be doing a 20 piece job and by the 4th or 5th shirt the colors would be different. It was just not consistent.
We always had a terrible time printing reds properly. Blues were also a problem. The white lines kept clogging out of nowhere (even when we were half way through a production, what made stoppage time a massive problem).
On top of this, the pre treatment procedure and drying times were always a pain. In fact, I believe that you can't do dark shirts properly (long runs) if you don't have a pre-treatment machine and a conveyor belt for drying. Most small businesses cannot afford to buy this. Some people charging $20 per shirt might have the time or the energy to endure with dark prints but doing long runs it cannot be done.
Since October we gave up on dark shirts so we're only doing white/light colors. I have to tell you it is the best decision we could make! We have NO PROBLEMS working with it all day, every day. Mondays are not a nightmare anymore (we would come back from the weekend and spend the whole morning trying to unclog the white lines) and a simple clean is enough to have it working. The profit margins are HUGE, the work involved it pretty much non existent (load in the shirt, press print, load off the shirt, press it). We're doing 150/200 shirts a day sometimes.
This is pretty much my experience with it.
Knowing what I know today I WOULD NOT buy the mP5i; even though it's a beautiful machine and it prints incredibly well, it's still a 25.000€ machine for printing white t-shirts! I am sure I could have bought a similar machine for half that much and would still be happy with it.
Different people in different markets might have different experiences; this is mine, the decisions I made dropping the white ink are my own.
We are moving into a new facility and we'll have a conveyor belt for screen printing; I am considering purchasing a second hand pre-treatment machine and give dark shirts a second try. It bothers me having this kind of machine only doing light colored shirts.