howdy. the '09 950 was my first bike and still my daily rider. i'll forego all the pros about the bike since you're already interested in it and probably know about them. so here's the main things to look out for:
belt issues - the drive belts on this model come notoriously too tight from the factory with manufacturer specs which calls for it to be too tight. so the first thing you're gonna want to do is check rear wheel alignment and belt tension and make sure it's no tighter than 5-7mm, leaning closer towards 7mm
front pulley nut - a few people report some loose or shearing front pulley nuts. you can easily check this by pulling off the front pulley cover and see if there is noticeable red dust all over the place
seat - factory seat is very uncomfortable for most people for any kind of distance rides
knocking sound - during the first few thousand miles of this model you will hear a ticking/clanking engine sound that is perfectly normal
recall - there was a recall on the early years for this model to replace a faulty fuel line or something. make sure it's been taken care of for yours by checking the VIN online or with a dealer
some others may chime in with some other small things, but those are the most common ones i can think of. other than that, fantastic bike. i've got nearly 100k miles on mine.