Yeah unfortunately it all depends on your situation.
For me Caltrain works great. In the AM I get on 99% of the time and have 5 options (3 main ones, and 2 for when I'm running late) at 7:40, 7:46, 8:01, 8:40 and 8:46. The 40's are bullets. I usually catch the 7:46 or 8:01 which are "limited express" and take an additional 7-8 minutes to get to SJ from Hillsdale. I like them since, you are basically guaranteed to get on, unlike the bullets. For the cost of less than 10 mins... no big deal.
In the PM, I get on at San Jose, so there is obviously no problem getting on. I get on at either 4:45, 5:05, 5:31, 5:45 or 6:05 depending on when I got to work in the AM, or how busy I was. All those listed stop at Hillsdale.
So Caltrain works great for me. However, it seems there are basically 2 groups that get screwed:
1) You live/work near a lower volume station, and get serviced less frequently.
2) You live/work at Palo Alto or Mtn. View, and ride a bike. In this case it's a battle to get on the train.