To reiterate,
I don't think St Andrews quite compares to an Oxford/ Cambridge degree but it would be about 1 step down on the ladder with the likes of Durham, LSE, UCL etc. (although universities mentioned generally rank higher) It's generally around 5th/6th on the league table (out of roughly 120). It's also quite hard to compare St Andrews to other Universities due to its small size, which means it has a lower research output than other larger top universities. However it does have an very good reputation for teaching, which is very important at undergrad level. (this is one of the reasons along with student satisfaction that it does so well on the league tables)
Depending on the course, I would say that it has very good graduate prospects and there are careers fairs for Law, and management and finance, if interested. For IR, The International Relations school ranks 2nd after Cambridge according to the Guardian University Guide. (although rankings have to be taken with a pinch of salt).
Prestige is hard to judge but it is a world-class university, it is the 3rd oldest university in the English speaking world, and it is very competitive. (7 applications per place). It depends what you class as prestige!
I'm a prospective student for languages and St Andrews was my 2nd option (first option was Cambridge). I've tried to give you a fairly unbiased perspective but I do feel very pleased to have a St Andrews University place!