i find dropbox is by far the best service in terms of all the niceties, but the tightest in terms of free space.
however i would never consider paying for space as there are so many different companies offering far more generous space for free.
the best other service ive found with a lot of space is box.com, they dont advertise it but there is a referral link (google it) that gives you 50 gigs free.
i got dropbox up to 18.5 gigs due to referrrals and the extra bonuses they gave in the past for using their photouploader, the scavenger hunt thingie, etc....
so i use dropbox for my photo collection, and box.com for everything else.
one thing i found useful also, is that documents to go allows you to link it to pretty much every type of account, and therefore you can use it to get files, but only file types that it recognises. so great for getting pics, pdfs, etc but doesnt do ebooks. i did email them suggesting they improve this, as it would make docs to go an even better buy, as you dont then need the individual apps but they dont want.
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1. Yes, you will need one e-mail address per account. I would recommend signing up for one and then using that to send "refer a friend" links to the other accounts as each referral you make gets you (and the account which signs up using the reference) an extra 250MB of space.
2. Yes, you can add and remove files as much as you want. Deleted files can be recovered online for a short while.
3. Yes, I have accounts with Dropbox, SpiderOak, Google Drive and iCloud and store different data in each.
Alec
small point on 1. to get the extra referral space, it not only needs to be a different email but also the computer you first install it on needs to be different. so if on your home mac, you create an account, then if you create another account on it, the referral wont work, but you could create an account, install on your work computer, delete it, and then you could use it on your home account and still get the extra space.
also, if for example you have a mac and use windows both through parallels and bootcamp, you can install it on the windows partition and it counts as a different computer. if you can be bothered you can also delete and recreate parallels partitions and all will work.