Why go through the hassle of getting 23GB of space for two years when you can get it for life, with 5GB per additional referral from Copy? I've been using Copy for awhile now and it beats Dropbox hands down. The UI on both the web and mobile devices are more robust (with embeddable links, per user link sharing and the like), it is just as fast if not faster, and the best thing, in my opinion is that shared folders do not fully count against your storage limit the way it does in Dropbox. For example, if there's a 750MB file shared among three people, each person's storage is reduced by only 250MB, not 750MB. The more that file is shared, the less the hit to one's storage as it is spread across the number of people sharing. Android app is awesome.
If interested, I have a referral link in my signature. I really think Copy is going to either kill Dropbox or force it to lower its absurd pricing and referral arrangement.