Couple of observations from my experience with Un-Du:
1. Be careful using it on white posters. I used it to take some tape off an 'It's Alive' daybill and the fluid bled out around the tape (as you expect it to) but it caused a 'ring' - like you might see on a bath tub. I put this down to the poster having some dirt in it which the Un-Du had swept up as it moved through the poster. It's only happened to me the once, but beware if you're working on a rare / treasured poster.
2. The Un-Du can leave a lot of sticky residue behind once the tape has been removed. A couple of extra applications of the fluid and a gentle scrape / wipe will usually sort out the problem, but don't get too enthusiastic - the Un-Du can make the poster fragile and you can find yourself losing a little colour if you're not paying close attention.
Overall, though, Un-Du is a great tool! I've rescued several posters using it that you'd never suspect ever had tape on them!