I installed Google Desktop a few months back, but didn't really do anything with it, mainly because I was viewing it as it is billed - a desktop search tool. Probably because I'm quite linear in the way I work (I'm male - I don't do multi-tasking), a desktop search tool isn't something I really need a lot. Sure I need to find online resources, but then I use normal Google, or search journal databases (if Google could sort out the federated search across multiple database problem, that would be useful). But now I have my new laptop (a Vaio - why did it take PC manufacturers so long to realise that just because we use Windows doesn't mean we like beige - Mac users need not reply), I took the time to configure the other parts. I'm now almost a fan, and usually I don't bother with client-based software much. I love having my main blogroll going continually - it's a small window so you couldn't have all blogs in there, but the people who post a lot it's worth following. I put some quick reminders in the scratch pad, I have a number of different Flickr photo streams, as well as the My Pictures archive scrolling through on Photos, I use the To Do list to organise my day, and I even check the weather (it says it is raining in Cardiff today, and a quick look out the window - yes, it's right!).
So it's very quickly become part of the constellation of tools and services I operate within. I also use Netvibes for some similar purposes, so here is how they compare for me:
Pluses - always present, embedded search facility, good range of tools
Minuses - can be a distraction (I keep jumping off to look at photos that come by, or read blog entries - not good when you are trying to write a project plan), slow to load (maybe a result of the indexing process), photo streams seem to get stuck showing the same pictures (particularly if it is loading ones from hard drive).
Pluses - tabs makes it much more structured, seems much quicker at loading RSS (postings that have yet to appear in GD show up immediately), potentiall richer tool set
Minuses - no desktop integration, need to visit site, not a search tool