Between it's lack of features, general clunkiness of the web app, inability to send from
non-@me.com e-mail addresses, and the lack of message syncing once the new message has been pushed makes Mail completely unusable for me.
Contact syncing is great with iOS devices, but since Mail is not something that I can use it kind of takes away from the usefulness of contacts.
Using calendar syncing with iOS devices and iCal is great. Everything works as it should and it is obvious that this is what it was built for. The problem comes up when you try to use it with Windows and Outlook. Try sending appointment invitations from Outlook and look at the mess that it creates.
Find my Device works great and should be a must for anyone with an Apple device.
Photostream has potential, but the inability to control your content on a micro level once it hits the stream is a bit ridiculous.
Docs in the Cloud is fundamentally flawed, IMO. Apple's disdain for the file system doesn't really make any sense to me. I like the fact that they want to make computers easy for everyone to use, but dumbing them down to the point where they want to eliminate the file system is a bit much. Docs in the Cloud is just an extension of this (and a poorly executed one when you take into account that the doc syncing doesn't even work to the Mac). Add into this that there have been many posts on this site of docs just simply disappearing from iCloud, and it makes me want to give this a serious look even less.
My overall impression of iCloud is "eh"....at best.