Report from a May 2005 abortive but still marvelous hike on the Tanner Trail, with some I hope useful additions to other reports I've seen about the trail:
We were scheduled for a four-day, up-and-back trip on the Tanner, but one of us had new-boot problems so two of us settled for an overnight to the saddle overlooking 75-Mile Creek, just past Stegosaurus Rocks (a lovely spot to camp). Everything I'd read about the Tanner sounded scary, above all the info sheet the rangers gave us. Meanwhile I'd read about boulders, lack of shade, etc.
All in all: Yes and no. (NOTE: The following applies only to the upper part of the Tanner, which most people feel is the steepest and hardest.)
The scary part is that the upper Tanner is highly scree-ish; you have to place every single step carefully or you slide. But there are two mitigating advantages. First, there are only a few big steps on the upper part, which means that section is at least easy on the knees--and easy on the ascent. Second, as far as I noted, falls might be limb-threatening but are not life-threatening. In other words, unlike some GC trails I could mention, a misstep might break something but isn't likely to send you howling off a cliff. (Again, I gather there is some stuff like that below the Redwall, and doing the whole Tanner down in one day is indeed hard on the knees.) For what it's worth: I slip on scree a lot, fall a fair amount, but I took it slowly and carefully with two poles and fell, lightly, once in two days.
The big advantage of the upper Tanner is the ascent. Because of the short steps, and because going up on scree is far more secure than going down, it was actually a breeze to get up, other than the usual effort of climbing. We actually ascended it nearly an hour faster than descended. (And after all, no trail I know of in the Canyon is the kind of hand-over-hand steep we Eastern mountain climbers are used to.)
Upshot: The upper Tanner is tricky to go down but I don't think life-threatening, and on the return it's a great hike. My out-of-shape partner was swearing all the way down and cheerful all the way up. I'd predicted that morning, "I don't know that going up will be the nightmare you think." I didn't actually know if that was true, but in fact it was. Meanwhile, there's a fair amount of riparian shade all the way down to 75-mile creek.
I'd do the trail again no problem, and recommend at least three days, four if you really want to enjoy it. The two-day, river-and-back people we passed coming up were mostly suffering heavily. Also, if you plan to stay a night on the way down, I'd recommend seven quarts of water per person, minimum. You can cache water for the trip out somewhere on the flat part. Note that if the Colorado is silty, as it was the week we were there, it'll jam your filter. Consult with a ranger about how to handle heavily silted water.
And note that all this presumes you have some serious experience with hiking. I'd strongly discourage a beginning from doing any of the Tanner. For experienced hikers it's nothing you haven't seen before---only perhaps a lot more of it.
Finally: Three cheers for Grand Canyon rangers! On the phone and in person I've never known them to be other than helpful, patient, wise, and personable---which is good, because you need to ask them lots of questions. (I got three pieces of advice about the silt and the ranger's wasn't the most promising, though we never got to try any.)
Seventy-Five Mile Creek Canyon