QUOTE (Thanee @ Feb 4 2011, 08:04 AM)

It has several new qualities (those are quite useful), lots of new metatype variants and rules for all the weird stuff (SURGE (basically "mutated"), vampires, shapeshifters, drakes, etc), advanced lifestyle rules, advanced contacts rules, some alternate character generation systems (one has to be changed a bit for SR4A, though), and some general information for shadowrunners (team roles and such).
Note, that you can buy the main books for a fair price in PDF format (SR4A $15, the other five $12 each).
I have both the printed and the electronic books, and since I bought the latter, I am pretty much exclusively using those.

Going to second the vote for RC.
The metavariants, build-a-furry surge rules, and vampires are kinda situationally useful. This is somewhat intentionally unfair statement - they CAN be good, and used properly, but some people hate them. It all boils down to what kind of game you're playing, and the group you're playing with.
The advanced/detailed lifestyle rules, Group Contact rules, and, most importantly, the qualities section are what makes the book for me. the main book just seemed light on stuff - like qualities - you could use to build flavor into a character. RC handily fixes this, and has tons of stuff that's just interesting for all archetypes.
Its also worth noting that they made a second printing a while back - Adam and Hardy said that some rules updates/errata made it into print. Get the latest printing if you can, its always a good idea. (this is especially annoying, because they haven't bothered to put the changes on the website).
I'd also recommend Arsenal. Even if you just use the guns from it, its a solid buy for all archetypes.