Why would you buy your instructor a pressie? You have been paying her/his wages for the last 50ish hours!? It's their job....
There are arguments for and against.
For: flying instructors are notoriously underpaid, and many are barely getting by. If you can afford it and if they have done a good job of getting you through your training with minimum time and headaches, it would seem a reasonable gesture.
Against: flying instructors are notoriously underpaid, and if people want to pursue this line of work, that's their own choice. If/when they land a lucrative airline job, will they refund any of the fees that they charged you?
Ultimately it is a matter of personal choice. If an instructor really puts forth a good effort (e.g., is constantly good humoured and encouraging), I think that it should be rewarded. Personally I didn't give my PPL instructor a monetary gift, but I sent a detailed letter to the CFI and said what a good job he had done, how professional he always was, etc. Hopefully that letter will be of use when it comes time to look for another job or to negotiate a raise in salary.
BTW, I gave a copy of the letter directly to the instructor, which is good because the CFI never said anything about it to him!