Commuting
My monthly pass for Caltrain is $59.75, which is a 33% discount over buying individual tickets. It normally only works between Menlo Park and Sunnyvale, the boundaries of Zone 3. (Caltrain has six zones, with Zones 1-4 more or less being SF, the North Peninsula, the Mid-Peninsula, and the South Bay.) On the weekends it works in all zones. Otherwise you pay regular price for a zone upgrade ($1.50). I wish the upgrade was cheaper if you had a ten-ride or monthly pass, but that's not one of the benefits of the passes. (If I had to guess why, I'd say because would make the upgrade process too complicated.) The passes that cover more zones work on other local public transit systems, but even though it's a great benefit, I don't use other transit so much that it's worth the extra cost (about $50 per zone). The total cost of my transit is less than most people I know pay for just gas every month, not to mention insurance.
It's pretty fast, though slower than driving if your destination isn't close to your transit drop-offs, or over a longer distance. For example, the slowest train from Menlo Park to Sunnyvale takes 20 minutes -- same as driving in medium-heavy traffic. But my commute takes 55 minutes on foot, because my work is 20-25 minutes from the station (and home is ten minutes). You can take your bike on Caltrain, and I have been doing so, because it shortens the commute. When I commuted from San Mateo, the train portion was 40 minutes, whereas driving that distance is generally about 30, so driving gets better time the further the distance.
Your schedule is less flexible than if you drive, which is fine for me, because I like a routine that forces me to be in the same place at the same time, more or less, five days a week, and my boss likes me to have a regular schedule anyway. The commute-hour trains are frequent enough that it was easy for me to find ones that made a good schedule, though when I walk or have errands to do in Sunnyvale I do take the first evening local (7:04p departure from Sunnyvale) and I wish it came a little earlier. Both driving and the train have some unavoidable variability. In driving, it's traffic; on the train, it's the train being late, for which there are many reasons. But Caltrain is fairly on time; I would say 80% of the time they're within 2-3 minutes of the schedule, 15-18% within 5-10, and 2-5% outside that time. That's less variability than driving, if I had to guess.
Caltrain is generally pleasant. Most people are just quietly passing the time on the way to work or home or fun. Occasionally you'll get someone with music on too loud or something. The seats are pretty clean. You can eat and drink, but there's surprisingly little trash on the train as a result. There is luggage space and a restroom, which is not always so clean but really great when you just have to go. I don't like how a lot of the seats go backwards; I prefer facing forwards. (They do it because they don't turn the trains around, so it's half and half each way.)
If you have a transit connection with more than one mode or agency, the hassle goes up a lot (more about that in non-commute transit). That's one reason I chose to live where I do, and one reason I feel so lucky to have found a workplace that's close enough to main transit to walk. I decided that that's how I wanted things to be, and I managed to arrange it, which works out very well.
I used to use SamTrans to go to Borders for work, and it was very functional. It was usually timely, which is impressive considering the El Camino traffic. I like that you can get on it and then pay; the stress of running for payment and then trains, for both BART and Caltrain, is kind of annoying. On the other hand, you have to have change to pay the driver, and I always had to worry about that except while I had a monthly pass (about $45).