Geometry Wars 2 Sales Age:
If your interested, GeoW2 has blown past the next fastest-selling game, Rez HD, in terms of first week sales (ie, 3 days, from Wed through Friday).
Thats the list of fastest selling titles as far as I've tracked. Geometry Wars 2 is at 36,000 and still has around 21hrs to go before I tally the final sales. So I would say that you'll find quite a few people to play against for Leaderboard domination :)