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I've given this same topic some thought, so I have some information you might find relevant to your decision.
I've used MobileMe mail (pretty much identical to iCloud mail) for several years after release, up until recently. While I really enjoyed the service, I ran into several issues that sent me to Gmail, where I will be staying for the foreseeable future. And these issues with Apple's mail service are still occurring, even well after the iCloud transition.
The major issue is spam filtering. MobileMe/iCloud has the worst spam filtering of any email service I've ever used. Despite having gone for the first couple of years with MobileMe nearly spam-free, in the past year or so spam mail getting into my inbox has increased significantly, and I was given a first hand look at how terrible Apple's implementation actually is. When you get spam mail, you have two options, according to Apple's web support (
http://support.apple.com/kb/TS4019): either go to the webmail interface (only accessible via desktop) and actually open the email to mark it as junk (thereby loading the mail and letting spammers know your address is valid), or you go to a desktop mail program and forward junk mail to an Apple spam address. So essentially, you cannot report spam from iOS as things are. On top of that, the spam filter still does not seem to learn from all this reporting. I've reported hundreds upon hundreds of very similar spam messages in the past year, and still I get up to 20 spam messages a day in my inbox (no spam has ever shown up in iCloud's spam folder, it all just goes to the inbox). Thanks to notification center in iOS, I'm painfully aware of the volume of spam. It boggles my mind that Apple believes that they can offer a supposedly "modern" email service with such a shoddy spam filter. Gmail can manage to watch the Junk folder on the server side, and when I move a message to the junk folder on iOS mail, Gmail actually marks it as spam and learns. Why can't Apple do this? If you use iCloud, you had better hope that websites where you use your address or people who store your email address in their contacts never get hacked, because if your email address gets on a spam list, you'll be in for a world of frustration. Apple Support is absolutely no help in this regard, and believe me, I've tried.
A second problem with the spam filter is that when some Apple filter does find "spam," the message is deleted before it even hits your inbox. Again, I've never noticed mail actually end up in the Junk folder. It is well documented on Apple's Support Boards that a number of people are seeing the spam filter pick up false positives, and legitimate emails are flat-out being prevented delivery. The iCloud users have no idea that this is even happening, and it is not until the sender asks them directly about the email that they realize that they are missing anything. This is also a big problem.
Aside from the spam issue, Gmail has better security for your account with features like 2-step verification and the ability to see a log of recent account connections.
Other minor issues for me with iCloud are terrible webmail, less security than Gmail, and less uptime.
I do want to like iCloud mail, I really do. I use all Apple devices and iCloud is just so easy to set up and use, and it works so well with the Mail apps on iOS and OS X. I use so many features of iCloud already, I'd love to go all in with it and use the mail service. But as it is now, I will absolutely not return to an Apple mail service until they fix the spam issue, at the very least.