OK this software doesn't cost too much so if you're a big user of iMovie and/or Garageband you'll be pretty pleased with the new features I should think. iWeb and iDVD get no new features. However, if you are a big user of iPhoto 09, be prepared to lose a lot of features you may like, or consider downgrading back to iPhoto 09 before the new version touches your photo libraries.
The good things I found out about iPhoto 11 are some neat new slideshows, playing video within the app (and slideshows), smoother book making and it seems quicker to manually assign faces. There may be other improvements I haven't discovered or don't use.
iPhoto '11 loses a lot of things like the ability to view or edit your photos full screen (the new 'full screen' mode is just a sort of iPad interface, not sure why, I'm still using a mouse and a keyboard). There is a lot of wasted space and inflexibility now due to panels being used instead of the old HUD overlay windows for info, effects, etc. It isn't possible, for example, to crop/enhance, etc while also seeing the file information. As soon as you require any info or edit controls you lose about a quarter of a laptop screen. You can only zoom in on your photo when the edit panel is present.
Quickly reviewing photos is harder - you'll need the large info panel open to see any file information because the small panel that used to sit at the bottom of the sidebar is gone. Another change in the sidebar is the missing count of photos in smart albums.
The application is no faster than before (except for the book-making interface, which seems better) and in some areas performance is much worse. Scrolling thumbnails used to be the benchmark for iPhoto but it seems like Apple have just given up and it now jumps jerkily from row to row. If you have inertial scrolling active you're going to get very frustrated. Dragging the scrollbar is even more staggered.
If you use the one-click email feature and your recipients like being able to add your photos quickly to their own library, you may be disappointed with the clumsy new built-in emailing feature. Of course you can drag the photos to the Mail dock icon instead, as long as you are not in full screen mode.
If you are familiar with OS X and rely on UI consistency to work efficiently, expect some surprises. iPhoto uses new non-standard visual candy, much of it inconsistently within the app itself. The contextual menus in particular are odd, but some of them will be good if you are partially sighted or if you are new to OS X.
Conceivably some of these problems (like the can be fixed with patches in the future but it looks like a lot of users will be forced to stick with iPhoto 09 or fork out for Aperture (and expect a bit of a learning curve with that option).
These observations were made on iPhoto '11 (v9.0) on a MBP 2.8 GHz C2D with 4 GB RAM and an 18 GB / 6000 photo library.