I've never used the service before myself, though I signed up when Google first opened it up (I like to make myself available, when possible). I just took the iPhoto plugin for a spin and it works well, though I don't think Picasa Web Albums is aimed at your typical Flickr user (its largest and most obvious potential competitor). Most of the social aspects of Flickr, such as tagging, groups, etc., don't exist in Picasa Web Albums - it seems much more geared to those who mainly want a place to upload their photos online, with a simple gallery that is easy to link family and friends to (it's also obviously geared towards Picasa users on Windows too). Users can chose whether to make uploaded albums public or private, which is where one of Picasa Web Albums' social features comes into play: users do have the ability to set other users as favorites in order to more easily view images and albums they upload (though, strangely, I can't seem to find any kind of public area where I can go hunting for other users). On the downside, however, Picasa Web Albums only offers a measly 250MB of storage space for free, with a $25 upgrade option for an extra 6GB of space.
All in all this is a handy service and the new tools are a great gesture on Google's part towards the Mac community. However, due to Flickr's overwhelming popularity (especially among Mac users) and a seeming lack of integration with other Google tools (where are my GmailThis! and BlogThis! buttons?), I think Picasa Web Albums has a little ways to go before it gains any significant interest among Mac users.