I'm a huge fan of racing games (my favorite genre) and I've played just about every major racing game you can think of, including pretty much every game in the Need For Speed series and I have to say...
Pro Street absolutely sucks. Of all the racing games I've played, Pro Street is the worst by a longshot. It's nothing like the previous Need For Speed games, the new "realism" is implemented horribly and the series definitely isn't ready for it and I'd avoid purchasing it by all costs if I were you.
You could probably get Most Wanted and Carbon together for about the same price. They are much, much better than Pro Street.