GXL  So it remembers the state of the full backup and it backs up everything that has changed since the full backup?

That's a good news - thanks.

BTW I haven't tried it yet - I based my comments on feature descriptions, so you might consider adding this into you product description :-).

The incremental backup would be more than usefull too so the files that already got backed up since the full backup and have not changed would not get backed up again-and-again in the differential backups.

Given that the hard work has been done on differential backups I believe it should not be too hard to add the incremental support together with keeping track of files backed up in each backup so the application would be able to prompt the user for the backup media to insert progressively to restore the files/folders to a desired state.  
(Version 1.0b7)

GXL  It is very positive that the developer went out of their way to pay so much attention to the online backup reliability and it's ability to recover the backup process from wireless or WAN interruptions that are so very common and can hamper the backups.

However, it doesn't seem to support incremental and differential backups.

It's quite an essential feature so I will keep an eye on this product and once this feature is added I will most certainly buy it.  
(Version 1.0b1)

GXL  The WhatSize is not free anymore so that argument can't be used.  
(Version 1.6)