I'm a little late to the topic, but I'll echo a lot of what was said. IDP is sooo much better to play than standard defenses. There are plenty of pros, and a few cons, but here is my perspective:
1- IDP allows you to build a winning team in so many more ways. As long as the scoring is equal between offensive and defensive players. For example, in our league, the top three to five players in each catagory (RB, WR, TE, LB and DB) all average about 50 pts per game. Outside of scarcity, this makes a trade of Bobby Wagner (LB SEA) for Antonio Brown doable. We also have found that the person who builds the most solid overall team ends up champ. Not the guy who drafts two of the top three RB's. In fact, we had an owner draft both Ezekiel Elliot and Leveon Bell...and ended up in last place because he refused to improve his IDP's.
2- We used to go all out and split out all the options: Defensive Ends and Defensive Tackles, Cornerbacks and Safeties; but what we have found is that the core fun is keeping it simple. Instead of using individual defensive linemen, we have a Team Defensive Line. The individual players are either LB's or DB's. We found that once the first couple of defensive linemen were off the board, it was a crap-shoot in drafting and week-to-week scoring, so that's how we ended up with the team DL. We also run a Team Offensive Line as well as a Special Teams, and their scoring is different from the individual players. The top ones average about 75-80 pts per game. It adds a good dynamic and makes drafting one of them early very viable.
3- It's harder to find good info on IDP's. We have two good writers in Jene and John, and their are a few others out there, but for each one of them there are a hundred other writers that focus soley on the offensive players. This is one thing I asked for more of in the survey Joe sent out earlier.
4- It's critical to know what system each team runs and if they have switched in the off-season. A 4-3 defense will a higher value on certain players than a 3-4 defense will. And then there are the teams that run mixed...and special packages. You really do need to do your homework here to have an edge.
5- The landscape changes faster on defense. Often, you can see the changing of the guard on offense- like an aging WR getting less and less opportunity. Defensive changes sometimes happen without any warning and finding out why is hard to track down.
6- The old adage is: Talent + Opportunity + Situation = Value. On offense, Talent is usually the determining factor. On defense, situation plays a much larger role. I think a previous poster mentioned that marginal players can sometimes provide inflated stats simply because of the role he plays. It happens every year and requires a little digging to find those gems.
7- One more thing to add, I have found that it's important to have one or two more defensive starters than offensive starters. There are more startable DB's and LB's than WR's and RB's. We have 5 offensive players (6 if you count the QB) and 7 defensive players as starters.
8- Almost forgot, the waiver wire becomes more fun. We award a single player on Wednesday of each week through blind bidding, then it becomes FC/FS. Having IDP's allows more top tier talent on the wire.
Those are just some quick and random thoughts. Good luck and I hope it works out for your league.