GMail's new category system* has definitely grown on me. There are a lot of things I miss from Outlook but auto-categorization is awesome. There is a small learning curve which can be shortened by reading the 1 sentence description of the new categories (from the desktop website - hover over them and +5 for simple discoverability in the User Interface!).
The more I think about it, the more it strikes me that this system has a very real advantage over the fixed rules approach. The problem with those rules is having to maintain them as they inevitably become out of date (distribution list name changes, email address changes, etc...).
The built in categories are great: Priority, Updates, Social and Promotions (aka spam-lite). For example: my apartment complex sends out a "delivery for you" email whenever a package arrives. I want to know about that immediately so I've dragged those into the Priority inbox. I've only had to do this 2 or 3 times before the system learned to automatically put them there. It somehow figured out that I don't really care too much about the "buy stuff from this vendor we sold ad space to" type messages (they go in Promotions).
When it miscategorizes an email it's easy enough to correct by moving it to the right category. This can be done on phone but I've found that it's faster for me to recategorize a bunch of messages at once on desktop.
I expect mass mail campaigns to optimize for (or against) gmail's new system. Given that it learns from me dragging and dropping stuff into the right category it should be harder to game the system for long.
Since I've given up on manually categorizing** high frequency information streams I kind of appreciate the balance they struck between broad applicability without overwhelming granularity. Once a list has 7 or more elements in it IMHO it becomes unmanageable for most and has to be generalized/massaged/abstracted back into some set of 7 or fewer buckets so that humans can manage it without too much overhead. Kudos for cutting that magic number in half (due to User Interface concerns I'd guess).
While it takes some manual categorization before to get the system from a totally anecdotal guesstimate of 67% accuracy up way above 95%+ accuracy the up-front work is easy enough and infrequently required enough to not outweigh the substantial benefit of having a personal software agent helping me sift the wheat from the chaff.
*-I'm a feature-oriented guy and this is personal email so I'm not terribly concerned about privacy implications.
**-manually priming (supervised learning training phase) is not to big a deal if it doesn't take too long and doesn't have to be done more than a few times.