The other reviews here show that this is obviously a Marmite set - you either love it or you hate it. Me, I love it I treasure it and play it often. In such circumstances another review may not be of that much help to an undecided prospective purchaser because you can't know in advance whose taste you share, but for what it's worth:
The treasure of this set for me is the B Minor Mass. It has been described as the greatest work of art in any medium ever and while that may be a rather overblown claim it isn't wholly indefensible. Gardiner's interpretation does it proud. It has intensity, intellectual weight, a deep respect for the work and, above all, radiant beauty. I love it. I also have the Andrew Parrott recording (now available very reasonably on Virgin Veritas) and treasure the two equally. I've heard quite a few others which I don't like as much. There isn't a single correct way to do the B Minor Mass and your taste may not coincide with mine, but if you're looking for a really fine recording I don't think you can go wrong with Gardiner (or Parrott, for that matter).
I'm less qualified to compare versions of the other great works in this box as these are the only recordings I own and listen to in detail. Probably the reason for this is that I'm extremely happy with them. I've heard others which I have liked very much, and the Gardiner versions seem to me to stand up with the best of them. The singers in this set are simply fantastic - Nancy Argenta, Anne Sophie von Otter, Barbara Bonney, Anthony Rolfe Johnson, Stephen Varcoe to name but a few. The orchestra and chorus sound fresh and involved, and I derive huge pleasure from the whole set.
Gardiner seems to arouse strong passions in some musical breasts - one Amazon reviewer has written scathingly of both his Santiago discs, for example, which I think are sublime. For myself I think he's a great Bach interpreter and nine discs of him directing some of the world's finest music at this price seems to me to be a monumental bargain. I can see that some knowledgeable and passionate music lovers here have grave reservations but I, like the great majority of professional reviewers did, recommend this set wholeheartedly.