MobileMe is the best service for push email... hands down. I've never had a problem with it. I get my emails instantly and it's great. I'll place a bid on ebay and seconds later, I'll get the confirmation email. It's really awesome! Even when I buy stuff at the Apple Store, they email the receipt to me... I get the email before I can walk out of the store... as I'm turning around to leave... I get the famous email sound "whing!"
Yes it definitely works good, but the problem I have with MobileMe is that it is very much a one way street. Specifically, if you look at calendars, MobileMe doesn't allow you to use offline software (i.e. Mozilla Sunbird) to view, edit, etc, the calendars. The only solution is to use a number of different software together to actually enjoy the benefits of iPhone syncing calendars on the desktop (without going to me.com), and even that hardly works well at all. So while I'll grant you that MobileMe push works great, I think the basic problem with the service in general is that it is very much a closed environment. Obviously this is partially simply the nature of Apple in general, but for a service that you're paying for, I think one should be able to access and edit its features in a variety of ways, and not simply via a browser (which I still find inferior to a regular desktop application).
That's why I think a lot of people like Google's approach better (sans push, for now). They are very much an open platform in many ways, and each aspect of the system, mail, calendars, etc, works superbly with a variety of clients (desktop, mobile, etc). Since Google's approach is to spread their services as quickly and effectively as possible, they design each service to work in a variety of ways. The result is that the end user enjoys the benefits. That is why I hope that push comes to gmail - while I still use MobileMe, my use of it is very limited because of the confines of its architecture. As such, while I use MobileMe to improve my iPhone experience, I could never see myself relying completely on it. This, I suspect, is why a lot of people are hoping for push Gmail. If Gmail pushes, the iPhone experience is greatly improved, and people can continue to enjoy the benefits of what is, in many ways, Google's superior product suite (precisely because of its compatibility and openness).