5Dimes has been discussed in some depth here before:
http://derbytrail.com/forums/showthread.php?t=671
5Dimes offers a 10% extra payout on winning win bets and exotic bets at most big tracks. On the downside, they have odds caps: For example, between 50-1 and 55-1 on win bets, and 500-1 and 550-1 on tris. The first 50c or $1 of a tri bet is uncapped. (depends on size of track). The first $20 or $100 of a win bet is uncapped.
There's more info in the thread above.
--Dunbar