Have to change my review today: 1.5 crashes. Asked the developer to send me an old version: They told me they "do not keep old versions". Well, well. This does not help if you end up with a non-functioning 1.5.1...

Great. Version 1.2.6 fixed a small bug with our service. Now you can use Twin to Backup all your data to our servers via WebDAV at http://www.cloudsafe.com/

The price is now euro 99 ($130 as of today). This is much too steep in my opinion because you also have to pay for the storage space. You could get almost three years of BackBlaze with unlimited data instead, and buy something else with the money you were going to spend on the storage.

That said, this app works pretty well with one significant major limitation: It only keeps one version of the data (even the MobileMe Apple Backup program will keep 30 days or so). I don't understand why this limit is built into Twin, because it's my storage space, so I should be allowed to clutter it up with old versions.

This seems to be the only application that can back up to MobileMe using encryption and incremental uploads. It's easy to use too (unlike Super Flexible File Sync) and is pretty stable (again, unlike Super Flexible File Sync). However, it's much, much too expensive for what it does.

I thought this program looked very promising and I really wanted to like it. Unfortunately I couldn't get it to work on my Mac to backup on a server over the internet. Despite multiple attempts of contacting the support team via support forum and emails I wasn't able to get a single helpful response within the last 6 weeks (!). So I paid good money for this program, haven't managed to back up a single file yet and am getting more and more annoyed with them. I tried to get my money refunded - also unsuccessfully.
Bottom line: Unfortunately this is not a program on which you can rely on for backups and the service and support they offer is absolutely appalling. Nice idea but nothing for serious use!

I use Twin to back up about 40 GB of data to Amazon Web Services.

Pros:

Data persistence. Amazon won't go out of business suddenly, taking your data with it. You don't need Twin to retrieve the data. If the developer disappears or the app stops working, you can still get to your backups with third-party tools.

Data privacy. Data is encrypted with the standard AES algorithm before being transmitted. You can decrypt the archives with open-source tools.

Stability. No crashes in several months of using this app.

Flexible scheduling. You can set Twin to back up periodically and/or when the network becomes available -- useful for portables that aren't always connected.

Cons:

Inefficient use of bandwidth. Data is packaged in archives of fixed size, which is decided at setup and can't afterwards be changed. The default is 1 MB. So if you change a single byte in a small file, at least that amount of data has to be sent. If you change a single byte in a large file, the whole file has to be sent. It would be better if Twin used something like librsync to back up only the parts of files that have changed.

Mirror backups only. The backup contains the most recent version of each file you've selected. There's no option for incremental backup.

No obvious warning of failure. The interface is normally hidden. The only indication of a failed or incomplete backup is in the interface. If you don't look for it, you won't see it.

Suggestion:

Offsite backup is a last resort in case of total destruction of your primary site, including your local backups. Put all the data you need to recover your backups on a small USB flash drive (encrypted, of course) and keep it with you at all times. That would include, at a minimum, your AWS public and secret access codes, your backup encryption keys, and your Twin registration data.

1.5 crashes. Please fix.