As for planning the trip, I just can't imagine seeing the falls now in their "trickle" condition. There's just something beyond overwhelming about standing on that steel canaopy near the brink of the Devil's Throat. And, so many of the smaller falls along the way aren't there (at least according to the pictures I've seen). That's what makes any number of trails in the park so cool....around every turn there's a beatufiul waterfall, and benches for sitting & enjoying it. All of the trails are the kind where you really find yourself making several stops along the way to take it all in. If anyone has Iguauzu plans now, I'd suggest doing anything you could to postpone them until the waters "return.
Note: The park does do "moonlight" walks, where you can see the falls by the light of a full moon. Apprently, those are awesome as well. Dates for these can be found on the website for the park.
I wouldn't waste money on a guide...there's a lot of interesting stuff they tell you, that really does enhance your experience of it all, but I'm sure a little homework would find it all out and save the $$.
I would blow the cash for the boatride under the falls: You and everything on you will get incredibly wet, as yes, you get to go under a few of the falls, and you go under them several times, for all those "camera angles." I bought a water-proof disposable at the hotel gift shop (which, doh, I should have bought at Target beforehand). When the driver points the raft straight at the Devil's Throat, then pauses, leaving everyone strangely silent thinking "Oh...my...gosh...are we really going in THERE????!!" Then he guns the engines and---whether or not you actually plunge "The Throat" is something you'll have to find out
The boat ride throws in some cool rapids running down the river, and all in all it's a safe, you'll live, adrenalin rush like no other, and it ends with a cool little "Indiana Jones" style jeep ride thru the jungle. The views of the falls from their "bottoms" where they meet the rivers is really amazing, and worth the trip...it's "thrilling" kind of adrenalin rush, not the "I'm gonna' die" scary kind.
The falls view room is great thing. I was upgraded just for being "Gold." Gold also got me free internet access, and the HUGE breakfast buffet in the mornings. Back at the room, you can leave the screen door closed, the glass door open, and slumber to the sounds of the falls. You can sit out on your balcony at night with a nice glass of Argentine wine and listen, listen, listen. And, since you really should be up early anyways to get into the park before the tourists come, you can see the place at sunrise...awesome. Something really cool about just chilling out on the balcony with your morning cup of java and the falls right there. But--the balconies on them are not private at all, if that's important to you. It's a "pyaramid" kind of shape, and so anyone on the upper floors have great unintentional "views" of the lower floors' balconies. And, if you're on one of the lower floors above the bar, the bar has an outdoor terrace where people can stay and chat until the wee hours. Is the hotel expensive? Sure. Is it as nice as its price? Heck no..don't expect 5 star...but as others have said, it's no dump either and you really can't beat the location. You'll be glad you did.
Rent the movie, "The Mission" before you go. Jeremy Irons, Robert Deniro. It was filmed there. Just like Ski bums watch the ski flicks to get them excited for a trip, the incredible views of the falls in the movie will get you in the mood, while teaching a lot of the history of the area. The soundtrack is beautiful, worth a download or two.