To be honest, they aren't very good. People will tell you they're great because they are from some other place in Texas where they're worse, but they aren't metropolitan-city quality, maybe small town quality. I used to complain about the T in Boston because it was old smelly and crowded but oh man, it's lightyears from anything here.
I do hope that public transportation is getting better, and it's worth trying to buy near bus routes and rail in case they get used more, but right now most middle-class people don't use public transportation to commute.
We deliberately chose to pick a house that's near bus lines (in Brentwood) because we believe in it, but there's no way to commute to either of our jobs easily with transportation even though both are less than 5 miles away. And both of us work for large offices, both for the state government. Because we both care about public transportation (he works for TXDOT), we go out of our way to use it for concerts and festivals and such but that's about it.
Let me give you an example - when I first moved here several years ago, I tried to make an appointment somewhere and asked what bus line it was on. The receptionist was really confused because no one had ever asked her that before.
Honestly the real issue is that Austin isn't very dense, which is going to make it difficult. For comparison, it has about 1/8th the population of Paris but it's physically 3 times the size. I don't see it ever having a really extensive rail system for that reason. And for some reason, there's a stigma with bus riding in much of the US, and I don't know how that's going to change absent some major shift. Which there may be with global warming and gas reserves and all that, but speculating on climate change is beyond the scope of the trulia boards, I think.