I just bought him for the "lulz". I can't seem to get him right yet though, which is unusual for me. One game I went 4-8-4, the next which it was 4v5 I went 5-8-4. Not up to my typical standards. Mainly I've been rushing Manamune, then.... Brutalizer?, and then I think I got as far as most of Frozen Mallet once. I also had a Vampiric Scepter bare in there somewhere.
Sivir on the other hand is amazingly fun to play against nukers mid. Just owned a Kass mid, whenever Null Sphere was cast, I just popped my shield and thanked Kass for the mana. I have a fun build for her too, I rush Chalice, then some boots, then go Madred's Bloodrazor, Warmogs, and Blood Thirster. Has worked every time, and I always have enough CS to get that far. Normally end up somewhere around 250-300.