Pros:
For my particular commute (home to Caltrain & back 2-3x a week) it has worked great - there's a rack across the street from home and two adjacent to Caltrain.
Perhaps due to the alleged low ridership, I have only had two instances of not finding available bikes; once when I left somewhat late in the morning (~ 11am), and last week when there were 7 bikes, but the rack was broken and wouldn't release them.
Cons:
As others have stated, the bikes are pretty heavy. It's fine on flat surfaces, but on slight inclines you will definitely work harder. For most of the non-hilly areas they're fine though.
General maintenance: I find the shifters often out of alignment and components like seats, shifters, handgrips seem to be wearing out faster than I would expect for relatively new bikes.
Rack distribution: This is being fixed, but right now, the sparse distribution of racks coupled with the 30 minute trip limitation (without paying more) means it's mainly useful for that specific trip. It's not practical, for example to ride 2 miles to Safeway for groceries since it would run past the 30 minute window to get it back to the originating rack. (Also the logistics of securing an expensive-to-replace bike in a bike-theft-heavy area).
Overall, if you have two frequent destinations served by the racks, it's a great alternative to Muni. Again, for my case, I consistently ride to Caltrain in ~ 10-11 minutes (tracked on the site) vs. 20-45 minutes on SF's abysmal (IMO) public transit.