[Assuming we are talking about Arlington ADP in case there are others.]
Tryouts - There appeared to be a fairly high acceptance rate (>50%) when my daughter tried out for U9 a few years ago. It was all scrimmaging after warm-ups, with multiple small-sided games (not that different from travel tryouts). They seemed to be looking for engagement on/off the ball and coachability (not that different for what they look for in travel tryouts but with a lower bar since this is a development program).
Experience - We were pleased with the program, in terms of level of play and the quality of the pro and volunteer parent coaches. There are 2 practices per week, one pooled with another ADP team led by a pro coach and another led by a volunteer parent coach. Games were usually on Saturdays on a field in Arlington. Most kids in ADP still play on their rec teams, so it is more like 3 practices/wk and 2 games/weekend. ADP doesn't keep score, and whenever other teams were short players due to illness, etc., there were also opportunities to informally guest play on other ADP teams. My daughter really improved over her 3 years in the program, made lots of friends, encountered different coaching styles, and was ready to make the move to travel at U12 (she wasn't ready at U9).