Having been there recently...
I would rate the terrain about a 4.5. To SAFELY do the hike, a good pair of hiking shoes is necessary. You can cross the streams in your bare feet but it's very risky due to the sharp and slippery rocks. We wore our kayaking wet shoes which were ideal. We saw many hikers double-poling across the streams. We only used one because that's all we had but found it sufficient with the wet shoes. Many parts of the Rim Trail are very slippery when wet which can happen quickly as I found out on my first visit there when a lightening storm blew through out of nowhere. As Dave mentioned, there are some places where hand stabilization is necessary...not rock climbing, but more of a descent or ascent than just walking up or downhill. A hiking stick is very helpful and we didn't meet many people without them. The trail is well-marked but very rough in places...primitive, as it should be. If you take your time and watch where you are going, you will be ok. One of the trickiest spots was just before arriving at the cache site. I found the coords to be as accurate as you can get...and that's using my Garmin 60cs, which has proven to be quite reliable in all 9 caches I've used it for. :D It only has 513 more to prove it's as good as my Vista. :eek: Regardless of how you rate it Dave...it's still a 10!!