Don't worry you'll love it. Last weekend I pulled all the old 105 gear off of my wifes bike and put all new Rival on (with the exception of a Shimano Compact Crank). I was very impressed on my first test ride of the bike and the wife loves it. If you are doing the build yourself you will find it just as easy as Shimano to install and easier to tweak. One thing I really like is how quick and sure the front dr shifting is. The double tap stuff takes about 5 minutes to get used to.
I am amazed to hear about the deal you got! That is smok'n!