I have been Commish of an IDP-only re-draft league for the past 8 years. It currently has 14 teams (started in year 1 with just 4), so every team plays each other in weeks 1-13. There is a 3-week playoff for the top 8 teams.
It is a Big Play league, with a custom scoring system that rewards Scoring, Turnovers, Negative Gains, and Stops in that order. The implication here is that tackles are greatly de-emphasized, unlike most mixed IDP league setups.
When I started the league, I tuned the scoring parameters so that each position contributed approximately the same Fantasy Points (on average) each week. It made for a wacky-looking scoring system, for sure:
Defensive Touchdown=25
Safety=10
Interception=8
Fumble Recovery=8
Fumble Force=5
Block Kick=5
Sack=4.5
Pass Defended=2
Tackles for Loss=2
Tackle Solo=0.6
Tackle Assist=0.3
Starters: 3 DLs, 4 LBs, 4 DBs, 4 Flex, 1 DEF
There are only 2 Bench spots, so team management through Bye weeks, suspensions, and injuries is crucial.
There is no prize money.
The primary benefit (excluding the social fun) is that is has encouraged a vastly increased awareness of defensive players in general, and high-impact IDPs in particular. This, then, has translated into an advantage in mixed IDP leagues, where managers still tend to be focussed on the offensive side of the ball.
13 of 14 managers from 2012 chose to re-up for the 2013 season. So I know the league has a high "fun quotient".
Hope this helps.