Class schedules aren’t identical (or at least weren’t when I was there), and travelling to the other schools can be extremely trying. There’s a bus system, but that doesn’t help when all road traffic is bottlenecked over the Route 9 bridge. You’re likely to eat up at least an hour getting to or from a class at another of the 5 colleges. Also, at least in some departments, you need to get approval before a class at another college can count towards a requirement in the degree program at your home school, adding another level of bureaucracy and paperwork to the process; if you’re trying to take a class at Amherst primarily because the equivalent class at UMass sucks, it’s unlikely to be approved.
I did my undergraduate degree elsewhere, and tried to take graduate courses at UMass in the computer science department while working on a masters degree in music. I was not impressed with the department, I was really unimpressed with the TAs, and if the caliber of the few computer science undergraduates I had regular contact with is any indication, you’re better off just about anywhere else. If you want an admission coupon to a Java code monkey job, UMass might be appropriate; if you want to study computer science, look elsewhere.
(But if you want an admission coupon to a Java code monkey job, you’re still likely to be better off getting a degree in something different and minoring in computer science, then getting Sun-certified in Java on your own. This will put you head and shoulders above the mass of recent college grads; you’ll have shown an aptitude for domain-specific knowledge by majoring in something else—and if it’s a soft science or one of the humanities, you’ll have “management material” written all over you—and you’ll be Java certified besides.)
In the interests of disclosure, my experience there was a good 7 or 8 years ago, and they might have improved drastically; but with the usual political games over the university system and the attempt to gut Amherst and move it to Dorchester, I don’t think that’s likely. Also, the traffic patterns haven’t improved at all.