I got this as a present about 3 years ago for my 18th birthday and I treasure it, it's one of the only sets of DVDs that I will never let leave my house :).
I'd say it's definately a must for any fan. I used to love it as a kid when it was all re-launched in the early 90's but I only had a few episodes of it recorded from the TV on VHS and it was very rare that it would ever come on TV at the times I was watching.
So having every single episode at hand, is the greatest thing ever. Even though it's less convincing now I'm an adult, it's still breathtaking how well the models are all made and how well the stories pan together. I think Thunderbirds is easily better than Captain Scarlet (although I have the CS box set too) because the stories were dragged out and in depth, rather than like in Captain Scarlet where they had to keep the episodes short so the story jumps about and the mysteries are conveniently solved with no real quests.
In this box set, you get 2 'book style' dvd cases, each containing 4 discs, with all of the episodes which were aired on TV. According to other sources, the episodes aren't perfectly in order but more or less follow how they were released to the public originally.
The picture quality has been enhanced greatly and although it's all in letterbox and requires a bit of stretching to fit on a widescreen TV, it still looks incredible, fully stretched on a 42inch LCD.
The sound has also been remastered and you can now experience the marionettes' adventures in full Dolby 5.1, which is incredible :).
The box set also includes a bonus DVD, containing interviews with the cast and creators and some behind the scenes footage.
All that is not included in the box set is the feature length Thunderbirds movie (Not the rubbish one made recently with real actors - an actual puppet one from the 60's), called 'Thunderbirds Are Go! - The Movie'... No biggie though because you can purchase it here for quite cheap.
The only thing i'l be looking out for to add to my Thunderbirds collection is this brilliant 1 hour long documentary which aired around Christmas 08' time, containing an in-depth look at behind the scenes and the history of the creation and success of Thunderbirds, featuring the creators and cast.
It's basically a 100x better version of the sloppy, non-informative DVD included with the box set... I hope the one from TV gets released on DVD :).
100% recommend. A MUST for ANY Thunderbirds fan.
Would even be worth the £125 retail price, easily.