Longmont is kind of far. If you're looking to live somewhere a bit cheaper than Boulder, I'd suggest Broomfield, Gunbarrel (North Boulder), or Lafayette. My fiance and I are currently living in Broomfield, which is about 15 minutes to campus on 36. We bought a house, and are really happy living in between Denver and Boulder. Longmont is a bit farther, and especially in the winter, can be a really tough commute if it's been snowing and the roads are somewhat icy.
Before we bought the house, we lived in Gunbarrel, which is basically just North Boulder. It was nice, because it was only about 10 minutes to get to campus.
Another place to check out is Lafayette. It's a nice little suburb too, and again, not too far from campus.