I've had a Flickr account for a while, but honestly, I don't use it nearly as often as I thought I would when I first set it up. I don't think that speaks to any problems with the site itself; it's really just a symptom of my overall laziness about organizing my photos. I'm notoriously bad at keeping track of photos I take, and always have been. I thought that going digital would make it easier for me to stay on top of everything, but alas, it hasn't.
Still, there's a lot I like about it. I am diligent uploading photos when I want to share them with other people, and I think that's a real advantage to photo sharing sites. It's certainly much easier to create the album and email a link to people than it is to attach individual photos, or to print them up and actually mail them. I also appreciate that important photos (like those from my wedding) can be uploaded so I don't have to worry about losing the CD that they're on, or what would happen if my computer crashed. There are also benefits I realize more as a viewer than sharer of photos - I have friends who upload lots of pictures of their kids to Flickr, and I like that I can then peruse them at my own leisure (as opposed to being ambushed with stacks of prints at lunch, happy hour or anywhere else when I'm really not all that interested).
I have an account on Photoworks as well, and have to say I prefer it. It seems a little easier to do the dragging and dropping to place things in albums, and I like some of the products and projects you can create through the site. In fact, when I got married, we had a photographer give us all the prints on CD, and I uploaded them all to Photoworks. Then I was able to order the prints I wanted, and I also was able to make albums not just to email, but real ones - with pretty covers and captions and everything. I was amazed by the options that were available, and loved how hands on it was. Looking at the site now, it doesn't seem that they have that easy blog button that Flickr does, but in all other respects, they seem pretty similar.
I'm a little wary of sending students to Flickr to search photos, for a few reasons. The obvious is that there's all kinds of stuff on there that I don't necessarily want students to think I'm encouraging them to look at. Additionally, the tags, captions and labels are only as reliable as the photographer or site user who uploaded them. Just this week in my graduate class, a student used a picture she'd found online of an author in a presentation, and our professor pointed out that the picture was absolutely not of the author she was discussing. Pretty embarrassing. A lot of students, despite their internet savvy, aren't well-equipped to evaluate how credible a source is even when the identities of the webmasters are transparent; photo sharing sites, allow users to remain much more anonymous, so that evaluation is even more challenging.