It DEFINITELY depends on the time of day. We go every year in early October when the parks are fairly un-busy. However, Rainforest often has very long lines for dinner from about 6 or 7pm on.
at night the waits are pretty long....the last few times we went, it was about a two hour wait but then we just hang out in downtown disney til its time. during the day its not that bad, once we just walked in!
I would put their number in your cell phone and call a bit before you get there. It can't hurt. We have had both long and short waits without calling ahead, but we called for breakfast one morning (I believe we called the night before) and got right in. Our food, however, took forever to arrive. But it was yummy and fun.
I would definitely make a reservation if you're going in the summer. Last August, the wait was upwards of 2 hours. The first time we went, we had a reservation for 6:30 p.m. We were still given a beeper but it went off after about 15 minutes. We happened to be in DTD again later that week and decided to check and see if we could get in. There was a 2 hour wait to sit indoors (where all the cool stuff is) but only 30 minutes to sit on the patio (no theming at all but the same menu). We opted to sit on the patio since we'd eaten inside already that week.
I think it's worth making a reservation is you know you're going to want to eat there.
Make 8:30pm reservations for a day when Disneyland closes at 8:00pm and then laugh at all the people who will be waiting in line for over an hour as you stroll past by them!