Mudman, given unlikely ideal conditions and theoretical perfect mechanical efficiency of a pump, no loss of vacuum in the bag etc., then the maximum pressure it's possible to exert will be what gravity provides.
Therefore, if gravity is about 13 lbs per square inch at sea level you could theoretically achieve 1872 lbs of pressure per square foot-- which is a little more than Axminster claim for their system.
Given mechanical inefficiencies it seems doubtful to me that a vac-bag of any sort could achieve anything better than perhaps 11 or 12 lbs per square inch. But that'll be more than enough to do a bit of veneering, and we certainly find it effective enough to achieve some quite complex and large laminating jobs out of plywood, MDF, solid timber, etc..
The biggest cause of problems in my experience is not getting the pressure on fast enough. On complex glue ups you're fighting the cure rate of glues and you definitely want pressure on fast.
Do whatever you have to do to achieve that goal.
As to the actual bag, these can be made to suit any project from heavy duty plastic such as that used under concrete foundation slab pourings, but the pre-made offerings are handy.
I've seen reviews of the Lee Valley hand cranked offering mentioned by another contributor, and I seem to remember that they are mixed, but I have no personal experience of using their system. Slainte.
