Cosmo Dog Park is a great park! I have been there about 5 times in the past 2 weeks and I have fun every time I go. There were some reviews on this park being dirty, but I have not seen that. The main area (not the lake portion) doesn't have a lot of grass, but its not really a big deal. The timid area has really good grass is kept up very well, although some days there can be puddles by the hose. It seems for the most part, that the people at the park (well in the timid area) are responsible for their dog, and I haven't seen one "real" dog fight and most seem to pick up after their dog. This is definitely my favorite dog park.
FYI: Starting in July, there will be a "rangers" patrolling the dog park to make sure people are responsible and clean up after their dogs. Link can be found here : http://www.azcentral.com/news/articles/2009/02/18/20090218parkrangers0218.html