I've been using GMail for almost a week now.  Enough time to use most of the features.  I don't have quite enough messages in it yet to need the search features, but I have played with them.

The Good:

• Very slick and easy to use. the interface is very simple and VERY responsive.
• The whole thing is driven with javascript.  In fact, if you attempt to view the source, you pretty much get nothing except javascript.  Very impressive.
• They appear to be using a webservices type architecture to download data in the background as needed.  Very rarely do you get a full refresh.
• The way it shows your messages as conversation threads really helps you see the conversation.  It shows your own replies as part of the conversation.
• If anyone is worries about it being more limited than something like Outlook because it's web-based, don't be.  the Outlook developers could learn a thing or two here.
• The keyboard shortcuts are very impressive.  I haven't seen a web interface make such good use of shortcuts.  I'm not sure how they've implemented it.  You can do all kinds of crap with them.  pretty much everything.
• The ads do not get in the way at all.  In fact, if you're looking for them, you'll have a hard time finding them.
• I find the concept of “labeling“ instead of filing to be very natural.  Items can have more than 1 label.  They're alot like categories.

The Bad:

• No signature support.  I don't really ever use them, but this sure seems like it's missing considering the thought they've put into other aspects.
• No HTML support in message composing.  I've got a very rich interface for formatting and styling for my blog entries, why not for creating gmail?
• Poor HTML mail rendering.  I've received a couple of messages that didn't render correctly.
• What am I supposed to do offline?  Of course you could argue this is a different model, but I'll have to continue pulling my mail in Outlook to have offline content.  I'm just not used to the model yet.
• When you create filters to “label“ messages (there aren't folders), you can't execute them on items currently in your inbox.  You can “test“ the filter by allowing it to show you what items match your criteria, but you can't seem to apply them.
• The view cannot be sorted.  It's stuck on newest to oldest.  I suppose the argument is that the searching and labeling removes the need for this.
• The search doesn't work on partial words.  Maybe I don't know what I'm doing, but if I search for wood, it will not match woods.  This seems pretty limiting.

Maybe I'll think of some more later.  If you have some, or have suggestions on working around the bad, leave a comment.