I love the ladder format. I play in a pretty active one that has four seasons a year. Being in Colorado, the winter ladder is pretty dead, but Spring/Summer/Fall are all busy.
If you're interested in their structure, here is their flyer for the spring: http://www.gatestenniscenter.info/pd...spring2013.pdf
In three or four years of playing, I can say honestly that I've never had a direct challenge. Basically, everybody's email is public and people send out emails to the group that says, "Anybody looking to play tomorrow, etc.?" I think the summer 4.5 group had about 50 guys on it, so I almost never had a difficult time finding matches on short notice. The end of each ladder season is concluded with a sanctioned tournament, which is pretty fun.
The pick up times have not been very successful at the 4.5 level. There is usually one or two 3.5/4.0 players there, but I think that's hit or miss too.
I have met some nice people that are good sports playing the ladder. My experiences have been very different than a lot of the stories people post here related to leagues. So I'm not sure if a ladder is more laid back or not. It certainly feels pretty darn competitive out their on the court, but pretty drama free.
The only change I might make would be to do something to ensure that people play a certain amount below them as well as above them. I haven't really noticed it much at 4.5, but my first year back from a long hiatus I started at 4.0, and it seemed like there was a little group in the top 5 that only played each other (sometimes multiple times). That was annoying because they were only top 5 due to the luck of the draw at the start of the season. So they were gaming it a bit to stay there and as soon as the end of season tournament came around, those high seeds got smoked right out of the gate.
All in all, I really like the ladder and will be playing again this spring.