If two stars means "Meh. I've experienced better." then I agree, but I hate to be too hard on them despite the issues noted below, so I'll nudge it up to 2 1/2 stars.
On time, ok bus service. It's on-time because they allocate ample travel time to Chicago with normal traffic, so that may mean delays at intermediate stops to stay on schedule. The busses are clean. Wifi -- on those busses that have it -- is usually not working. There is often confusion when changing busses (some routes have you change busses at some point during the trip). Terrible web site and awkward online payment. Sometimes irritable staff. (Maybe they get tired of dealing with confused passengers.) It may be a good idea to get to the bus 10 minutes early to resolve problems. This is probably the only bus going from UW Madison to Chicago, so good or bad, you're stuck with it.
Note that the online payment system is so awkward I recommend waiting until you are boarding before paying. (You used to be able to buy tickets at the Union too, and if you're a member you get a discount.)