Highly depends on where in SJ<>SF you'll be, but over all I'd say it's pretty difficult for a few reasons:

- Caltrain currently terminates at 4th & King, not exactly a prime spot in SF, so you'll likely want to transfer to MUNI, so add that to your commute time and expense.

- The Caltrain ride itself is over an hour between SF/SJ.

- Caltrain lead times are't great outside of rush hour.

- I live in the middle of SF and it's 2hrs door to door to Diridon (SJ).

It isn't difficult if you're looking at the occasional trip to SF, but as a daily commute I would not recommend. I know quite a few people who do it and it's not pleasant spending that much time on a commute.