4.0.10 link: http://www.interspire.com/forum/showthread.php?t=14072
5.0.6 link: http://www.interspire.com/forum/showthread.php?t=14071
In terms of bug fixes there's quite a few in there, and for me in particular the addition of 3D secure compatibility with SagePay / Protx was very welcome.
Probably the main thing to note is this for 5.0.6:
This indicates they've tightened up the coding framework and styles to follow a set structure which should make things easier to understand for developers and mod authors. Unfortunately it also means that some mods and fixes may be "broken" so be wary of this when updating your 5.x code.Due to our efforts to standardize our internal coding practices this release affects the majority of the PHP files in Interspire Shopping Cart