It's great for a number of reasons:
1) It doesn't weigh down your BB with loads of email in the core system
2) You load it up when YOU want to read your email, not when the email comes through
3) Closing it is a nice way of forgetting about work
4) You could search all your old mail
I actually got stuck in Madrid airport without BIS quite recently, so I tried the mobile web version of gmail. It was a terrible experience - it was unresponsive, the ui was too big, yet too cluttered, waiting for 2 pages to reload every time you wanted to look at a new email etc etc was a pain. And this was on wifi on Blackberrys latest and greatest (9900)
Long story short I will miss the gmail app. IT wasn't great, but it had a lot of plus points. Sure, I can got get the gmail plugin and set it up via the native app, but if I wanted to do that I'd have done it in the first place.
I've only recently converted to blackberry, I'm a fussy bugger about keyboards and it really can't be beaten. This makes me sad.