My team has always done a variety of cross training exercises, but we added more to the mix 2 years ago. This year, we won our league title in 3 of the 4 divisions competed in and missed the league title by 2 points with varsity boys.
The only potential downside is that you add too much cross training and it takes away from your ability to do the actual running training that will make you fast.....ie if your legs are dead from tons of lunges or what not, then you can't run very well in your running workouts. This has not really been a probem for us except when we change the stimulus or add new exercises that brings in new and exciting types of soreness.
Where the cross training has been a real benefit is with my beginners. Early on, most of my newbies struggle just to run a couple of laps around the track without stopping. Add to the fact that the temps in Hughson are usually mid 90s or above for the first month of practice and it becomes very hard for me to get kids to "just go out for 30 minutes easy". It is always a struggle to get beginners to train enough. Either you risk injury/burn out and run them enough to be competitive or you play it safe and they end up kind of slow because their overall training volume is pretty low. I started supplementing my beginners training with about 20 minutes of cross training, done circuit style so that they are getting an aerobic stimulus. They do this 4-5 times per week early in the season and 3 times per week later on. The types of cross training are varied, but the constant is that the kids are having to perform multi joint, multi planar activites that keep their heart rates high.
I think I'll take some video of our group doing some of this cross training and post it to my team site. The only problem is that we are done for the season, so it might be difficult to get kids together to video them.