I quite liked it because it allowed me to choose all my photos first, then upload them. I didn't have to constantly monitor the site to know when I could upload the next photo.
Once uploaded I was given a choice of "styles" - currently there are 4 to choose from. My example uses the "wheel" style. You can also upload music, but I presume it really should be non-copywrite etc.
One difference with using One True Media for a slideshow is that you can also add in short video files.
Here's what my One True Media slideshow turned out like:
PS - to make it smaller to fit my template I changed in the html coding to height: 400; width 400. I could not find a way to resize my slideshow on the actual site. It may be fine being big with your template, mine is just quite narrow.
Slide is another similar site that allows you to upload your photos. Like Rock You it allows for several layers of customisation like:
* style (type of transitions - in my example I have used "revolver")
* themes (extras that appear in background)
* skins (which are a bit like borders)
* you can also select the size of your slideshow.
I uploaded the same photos to Slide as used above so you can see what can be produced.
Lots of scope for variety and customisation here.... happy slideshow making!