Its pretty good, but for $9.99 there should be a lot more and better tones available. Many of the tones are similar to each other (e.g. "Glass" and "Coins"), and many have a very cheesy Atari 2600 kind of sound.
When you assign tones by contact, it only seems to work for one email address per contact. Being able to assign by keyword would be nice, like in PushMail. The notifications also seem a little slow.
Overall its promising and will hopefully improve. Id give it 3 stars if it werent so expensive; as it is, 2 stars for an average app at a premium price.
The Mannerist about MailTones - Pager Alerts for Email and Gmail, v1.60