Thomas Brand on how to continue using MobileMe in Lion:
migrating to iCloud from MobileMe means losing the ability to sync Dashboard widgets, Keychains, Dock items, System Preferences, and data from third-party Mac apps that integrate with MobileMe
I never used MobileMe to sync widgets, keychains, dock, or preferences, so that didn't affect me personally. Other than mail, calendars, contacts, and notes, the only thing I synced through MobileMe was my OmniFocus database. And I knew I could switch OmniFocus to sync with the Omni Sync Server is that stopped working, so I jumped in and moved from MobileMe to iCloud.
It was a seamless process and except for a couple of hours of email downtime everything is working fine. OmniFocus sync is working as usual without me doing anything.
However, if you're using MobileMe and use iDisk to sync data (Yojimbo, for example), it might be better to wait until there's an alternative. Same if you have some Macs running on Snow Leopard or earlier.
Interestingly, I updated my main MacBook Pro (Lion 10.7.2, iTunes 10.5) and iPhone/iPad before migrating my MobileMe account over to iCloud. But I didn't update my older backup MacBook. That one was still running Lion 10.7.1. The next day I used it and my email was working fine. It seems a Mac running 10.7.1 logged into a MobileMe account still works normally even though the account has been moved to iCloud.