aftermarketsound, z-wave is a good choice if you're a DIY. Reliability may not be 100%, but it's darn close. UPB is also good, BUT you need a neutral wire at every dimmer. With Z-wave most of the better dimmers do not require a neutral. Some other Z-Wave devices do.
Just looking in the box to see if there is a white wire isn't enough, the reason being that some installations have a loop down from the fixture and the white wire carries hot back up. Not a great thing, but that's the way it used to be done.
Some will advise you of workarounds for the UPB neutral issue, such as putting a module up in the fixture, etc. That approach certainly works, but I didn't have an appetite for doing a bunch of rewiring, I just wanted to replace the dimmers and get on with life.
I first purchased UPB and went to install my first dimmer and discovered my white wire wasn't neutral. Fortunately, the retailer took back the UPB dimmer and I switched to Z-Wave. Z-Wave has been very reliable for me, and is very fast after using X-10 for years.
I think most will agree that you can't go wrong with either Z-Wave or UPB so long as you identify the issues of each and how they might affect you.