My library just took the extra step today of not only using Web 2.0 tools, but actually paying for them. We've been using a free flickr account for a little while, and just today bought a pro account.
But this wasn't a lightly-treated, "it's only $24.95," decision. No. Although the staff here who have been using flickr really like it (enough to max out the 3 set/200 photo limits of the free account), before we bought the pro account we evaluated other image gallery tools.
The others we reviewed were 4images, Coppermine, and Gallery Project. I found a handy Photo gallery comparison chart, which really supported the conclusions we came to: of the three, Gallery was the most consistent with our goals, but was too powerful (read: complicated) while not delivering everything flickr offers.
Flickr is easy, and is more than just a way to show off photographs. It is designed to share photographs, so they are easy for other people to find and comment on. That's what we were looking for - two-way interaction and participation.
The only real "con" against flickr is that it's a third-party service, and with them we lose the advantage of hosting our photo gallery on our server. We are now reliant upon them to be available and in business for as long as we are.
But, considering they just made an extra $24.95 today, I don't think we'll need to worry about their stability any time soon.