In fact, the only gripe I could come up with in using the service was the fact that the entire site runs on Flash. For people who get annoyed by Flash websites, know that it's not overdone. It's very smooth, very minimal, and loads very quickly. They're also building a purely HTML version of the site, so we can look forward to using that pretty soon too (I hope).
Granted, if you're looking to stash some nefarious data in a server and provide a download link somewhere (like a message board), this isn't the service you're looking for. It's only meant to be a point-to-point service; you upload a file that you want somebody to download, they get an email with a download link, and the transaction is complete. You get an email confirmation when the intended recipient completes the transfer, and they can download the file as many times as they please so long as two weeks haven't lapsed.
This simple screen is what you're greeted with when you visit the site. Add your file, a couple of email addresses and a note (should you please), and you're ready to upload.
This is what your buddy gets when he clicks the link sent to his email.
The page's background image changes regularly while your friend waits for the download to finish. All the images are pretty decent looking, and since they're all technically ads they support the process and make the whole service free.
Overall, it's a pretty nice way to handle large file transfers that can't be sent by simple email attachment, and since the download is good for two solid weeks you don't have to worry about somebody being away from their computer when you send the file.
WeTransfer makes file transfers not just painless... but kind of enjoyable