So far I've heard the term cloud used to mean web apps, but Apple redefined it to mean storing your data on the internet but accessing it through native apps. That vision is indeed compelling. Native apps have a far better user experience than web apps. In particular, the app is always available locally, as is all your data, and you can edit it without waiting for the network. Sync happens behind the scenes, as it should. This also means that they work offline.
And then there's Apple's penchant for exceedingly high-quality and emotionally satisfying interfaces. Just look at the MobileMe Sign In page, with the cloud slowly emitting small, delicate white puffs of data that get absorbed gently into the iPhones and iPads in the background. Steve Jobs overuses the term, but this really is magical (watch it in HD, by clicking on the Youtube logo):
But there's a fly in the ointment. Just visit me.com on your Android phone and you'll see this:
Notice that it tells you how to set it up on your iPhone, when you've visited it on your Android? I'm having trouble deciding whether this is arrogance on Apple's part or just gross negligence. They should have at least put up a screen saying, "Sorry, MobileMe doesn't work on your device."
Maybe it's Apple preference for building something absolutely great for 90% of their users rather than something merely good that everyone can use, as John Gruber puts it (he phrases it better, but I can't find the reference now). That works great for hardware and software. If 10% of mobile phone users find the iPhone too limited in some way, that's not a problem at all — they can buy other phones. Better to thoroughly delight 90% of your users. But this strategy totally fails for cloud services.
I won't use an email service that I can access from only some of my devices. The same applies to other kinds of data like calendar, contacts, notes, documents and photos, for that matter. If iCloud continues in MobileMe's footsteps, it'll be as interesting to me as Ping. Which is to say, not interesting at all. Except perhaps as a demonstration that Apple still doesn't understand how to build an internet service, despite the superficial UI delight.
At another level, none of this is surprising. Apple makes its money from hardware, so I wouldn't be surprised if they look at iCloud just as a way to keep iOS compelling in the cloud department. Whereas Google makes nothing from sales of Android or Chrome OS devices, and in fact gives the OSs away to drive traffic to their internet services, where they make all their money. So it's not surprising if Google cares deeply about giving you access to your data from all your devices, whereas Apple doesn't.
I do think Apple's strategy, if this is it, is narrow-minded — that most Apple users have non-Apple devices, and letting them use iCloud on these devices makes iCloud and therefore iOS more useful to users, not less. But I don't know if Apple's focus on building a wonderful product for the majority of users rather than a good one for everyone prevents them from seeing that. Maybe it's the innovator's dilemma.
So, if I have to choose between an elegant interface and access to my data from all devices, I'll choose universal access. Which means using services that, at a minimum, have web apps that work on all platforms. Even a desktop web app works on phones running Android, webOS, etc.
One step better is an internet service that provides a native app for at least some platforms, in addition to the desktop web app. Simplenote is an example.
But I'd like to not have to choose between a top-quality interface and being able to access my data everywhere. The gold standard is probably Evernote, with native apps for all the popular platforms — iOS, Android, webOS and even Mac OS X.
 Third-party apps are not good enough. I won't use iCloud if I can access it from Android or webOS only via a third-party app.