Best thing about free dining is it will save you money. Always best to compare a room only discount rate + food costs to a free dining booking @ rack rate. Depending on the resort an discount level either option can come out ahead, so best to do the bath.
The other "best" thing for me is, an upgrade to the deluxe plan from the free regular plan is very doable and I love the deluxe plan because I tend to try a lot of different restaurants and meals that I may not try otherwise.
As for as any "catches", other than having to purchase a hopper or water park add-on to your pass, which is really only a catch if you weren't planning to have it in the first place, I can't think of any. Dining plans are super easy to use.