As the resident VW and also GM guy around here, I believe I am qualified to speak to this...
Generally speaking, the value of a GLI depends mostly on the generation of the vehicle. Volkswagen has had a hit and miss track record with the car, that really takes it into some weird places, especially for me.
The holy grail of the GLI models is unquestionably the MKII-based version. Although it is not as technically sophisticated as the versions that followed it, it was about as pure of an experience you could get. The 130 BHP 1.8L 16V engine was one helluva sweetheart, and rowing it through a five-speed box, it really was something special for the time. If I could find an MKII in good shape, I'd buy it. Immediately.
The MKIII-based GLI technically does not exist. Although VW decided to mess with the GTI name a bit in the US, the GLI was replaced by the Jetta GLX VR6 which was by most accounts an impressive performer for the time. It was about as quick as the BMW E36, and all-in-all was a pretty nice place to be. It was far better than the GLI VR6 of the time, a car that seemed to be directionally lost outside of Europe.
The MKIV-based GLI was otherwise a return to form by Volkswagen, but ended up being a car limited by it's chassis. It wasn't until very late in the model run (2003+) that Volkswagen had the car properly sorted out, and even then, it was only the 1.8T model that really shined in the segment. Although the VR6 version was no slouch, with the power upped on the later 1.8T models, it was pretty much deemed irrelevant.
Moving on to the MKV version, it tends to be a little more hit and miss for me. While on the one hand, the performance is stellar, and you can't really ignore how nice the car is all the way around... It just seems a bit large, and a bit too grown up to be as rad as it should. Nevertheless, I'd still buy one, and over the GTI to boot. Skip the leather seats though, plaid is for bad-asses.
The MKVI, I think, is strangely compelling. Although the vehicle has been significantly decontented by comparison to it's predecessor, its still a pretty nice place to be. The 2.0T returns, as does a multi-link rear suspension, and that sophisticated subtlety that the model had in previous iterations. Although the entry price is a bit steep considering what you are getting, ultimately it is still one of the best performing cars you can get in the segment.
So, in general, the GLI is a great car. The problem is that it is not the best car you can get in the segment. I'd choose one because it is a grown-up's vehicle. Very comfortable at any given time, but able to take it up to 8/10ths whenever necessary.
As for it beating up on Mustangs, don't be a bloody pillock. I understand that you want to show up the kids with American cars, I did the same thing as a kid with my own Volkswagen, but you're not going to outgun a Mustang GT or a Camaro Z/28. It is reasonable if you do not like American cars, but take some time to learn about them. For the most part, German vehicles are not all they are supposed to be. Especially the products that come out of the Haus of Volkswagen.
(And that's coming from an American. 
