In my opinion when using VeSys AutoCAD 2006/LT2006 is the best version to use, it is the version I work with even though I have an upgrade to 2007 available (I use my 2007 to open a 2007 drawing and Saveas 2004 format ). VeSys only really uses the basic AutoCAD commands and there is almost no difference for VeSys users between versions. Below I have listed all of the Advantages/Disadvantages that I have found each version, this is by no means a definitive list it is only based on my use of VeSys and although I'd say I'm fairly good on AutoCAD I only ever use it for VeSys and occasionally sketching up plans of my house/garden .
Advantages of 2006
In my experience it is much faster to startup/load/install (compared to 2007 or 2008).
It has support for the 2004 AutoCAD drawing format which is faster to load, smaller etc (2007/2008 also has this support).
It has the nice colored inside and crossing windows compared to the earlier versions of the AutoCAD (2007/2008 also has this support).
VeSys generally works better on 2006 than the newer 2007 and 2008 versions (on 2007 VeSys closes the 1st drawing you open if you double click the dwg file directly).
Advantages of 2007+
Support for dual byte characters i.e. Chinese
I am fairly sure AutoCAD 2008 has support for Vista ..... just don't get me started on Vista I am XP all the way to the next version of Windows or the MAC OS!
Disadvantages of 2007+
Adaptive Grid - This is a new feature that changes your grid size as you zoom out, although some people may find this useful I setup a grid for a reason I don't want the CAD tool to change it as I zoom in/out - unfortunately there doesn't appear to be an easy way to turn this off and it is on by default. If you check the AutoDesk support pages and type in Adaptive Grid you'll find that the top question is "How do I turn this off permanently?"
New drawing format - AutoCAD made a big song and dance about the 2004 format, it was faster, more efficient, etc. 2007 has a new drawing format that doesn't appear to have any other benefits than maybe the support for dual byte characters/unicode, try for yourself looking on AutoDesks website for benefits. The other problem with this is users generally leave AutoCAD saving the drawing in the latest format which means it causes problems when they send drawings to users on older versions.
Hope these comments help!!!!
I currently use 2008. Hated it at first (change is evil, right?), but have grown into it. Sometimes it seems like Autodesk just makes changes for change sake. If it ain't broke, don't fix it people!... What is the deal with changing the plotting functionality on just about every release, c'mon...
Other than some quirky things between CAD and Vesys that are probably present with any version of CAD, I can't say that their is any real issues with 2008. I ran an evaluation copy of VeSys on my workstation until the eval copy ran out and am currently running an eval copy on a laptop that has 2005 loaded. I don't see any benefits/drawbacks to either.