I've fitted an VDO vision boost gauge along with VDO vision oil temp and oil pressure gauge in the spare din slot.
I think the VDO gauges are the obvious choice as they are fitted oem to vag/audi cars (or were).
The most difficult aspect of fitting the boost gauge is running the hose from the engine compartment to the interior, and that's not difficult !
Rather than tap into the upper return intercooler duct as suggested on the tdi forum how to, I fitted the the takeoff into the upper intercooler hose that joins the said duct and intake manifold together.
My reasoning was that :-
1, You can whip the hose off in two minutes and work on it off the car with no danger of any drill swarf getting into the inlet tract on the car... do you want to take that risk ?
2, To do a proper job as suggested in the tdi how to, you would need to take the duct off and the lower hose clamps are very inaccessable and would probably mean removing the bumber and headlight.
Bear in mind though that the hose I put a hole in for my take off point does cost 50 quid if things go wrong. I have no idea how much the plastic duct would cost if things went wrong on that but would imagine it would cost in excess of 150 quid.
Another thing to bear in mind is that whatever gauge you install, you'll more than likely get a loud 'chatter' noise from it from oscilations in the air stream (don't know what causes this ? maybe valves closing ??) But the cure is to fit a damper as close to the gauge as possible. Don't buy a specific damper that costs 20 odd quid, simply use an old fashioned inline fuel filter (as fitted to old BL Minis etc), that damps things down nicely.
A warning though, once fitted the gauge moves all the time and you'll find yourself staring at it.... a lot.... not a good idea really. One positive thing about having one is that you can try to drive 'off' boost, thats when you get best mpg and having the gauge you'll know when your 'off' boost. And for those that are interested, the turbo still whistles when 'off' boost so that is not an indicator.
One last thing... Don't buy a gauge that shows vacuum if its going on a TDI, diesel engines don't 'do' vacuum. I'd suggest getting a 0-2 bar gauge if you can get one, my VDO jobby goes 0-3 bar.
I'm more than happy to bore you all even more if you have any further questions..
Hope this helps.