I live in the Bay area and use BART pretty regularly, and even use Caltrain occasionally. I'm not sure if it's fair to compare the Light Rail to either of those transit systems since they cover a lot more area, but I think overall it's a pretty good system.
I like that it's clean (although it's fairly new, so I don't know how much longer that will last). I like that's it's cheap. I REALLY like the fact that on the weekends (it may even be the case during the week, I didn't notice) that it runs all night. Obviously it doesn't run as often in the wee hours of the morning, but at least there's a train every hour. That's always been a major complaint of mine with BART so I gotta give the Light Rail kudos for that.
The one complaint I have (like others have mentioned) is that it's slow. With most public transit systems, one of the benefits is that you can get from point A to point B faster than you would if you were driving. That's not the case with the Light Rail, however. I don't necessarily think it's that big a deal though. I would think in most cases, whether you're using it for work, going out to bars, etc., you would be aware of the schedule and would plan accordingly. I also wish it would reach other areas of the valley (right now it pretty much seems like a connection for Tempe & Phoenix), but it's new and hopefully things will eventually expand.
I also think think the whole "honor system" payment thing is kind of weird. I'm not sure if it's a good thing or bad thing. It's just weird. However, at $1.75 ($3.50 round trip) I don't see why you wouldn't just buy a ticket to be safe. That's VERY reasonable for a public transit system like this. I also think it's weird that there's just a flat rate no matter where you go. I know on BART or Caltrain the further you ride, the higher the cost is. Like I said, none of this pricing stuff is necessarily a good or bad thing IMO, it's just different.
Overall, I think the Light Rail is a good public transit system, and I'm glad that Phoenix FINALLY has something like this.