I've done this hike twice, and am going back to do it yet again this summer (2014).
There are a lot of good reviews on it.
I would like to add that if you have the ability to, you should hike all the way past Mystery Canyon and the Narrows into Orderville Canyon. The walls feel like you could almost touch them from side to side and a lot of the tourists drop out well before this part of the canyon, but it's not so secluded that you won't see anyone.
It's just not the narrows until you've hiked to "the NARROWS".
As others have said, this hike is best during a hot summer (correction, almost all summers are hot here) day. Check the visitor center to get a decent weather report on thunderstorms in the area as an added precaution to flash floods. In some parts, specifically the beginning, you have to wade through natural swimming pools with waterfalls dropping into it. Beat that Hollywood Hills celebrities (j/k)!
I've done this hike in sandals with no issues. Don't even think about bring flip flops. Just don't. But if you can get a better water shoes like booties, crocs, etc, i'd highly recommend it. When I first did this canyon as a naive 16 year old, all I brought were an old pair of Payless shoes. So they were soaked by the end. The friend I brought along was less than enthusiastic about it, but thanked me for not letting him turn back once we reached the end.