I can't imagine anything potentially deadlier than trying to put together an acceptable "complete Mahler" (1-9) with diverse oddball licensed product, especially given the blood-thirsty ferocity of Mahler performance critics and Mahler fans. The folks at Brilliant did a great job with this. I hope somebody got a raise.
I'm not going to slobber over the relative merits of each performance--I found them all good to great with some real standouts. The Vonk 2nd (despite so-so sound) and the Haenchen 6th totally threw me, also the Masur 7th which I liked a lot. Mahlerians are probably going to buy this for the classic Horenstein 3rd, or at least to have fun poking sharp sticks at the other performances. Newcomers wanting, say, a summer vacation drenched in Mahler would do well to buy this set and wait till a little later in life to be fussing over tempos and tuttis. For the price this is probably the best Mahler starter box you'll ever find. It's a terrific gift. Please go for it.
Newcomers should also be wary of getting advice from die-hard Mahler fans. They're a stunningly over-opinionated bunch and no two ever seem to agree on anything, even on what to order for lunch. That sort of says something about something. Fact is Mahler--a late 19th Century artist--was a recent recording era phenomenon and it wasn't until the stereo 60s that his career really started to pick up. This was new music for many, including orchestras, audiences, and conductors, not that long ago. Everyone had their own ideas and there weren't a lot of precedents. Also, fact is that despite Mahler's precautions (his performance instructions are perhaps too detailed) there seem to be many satisfying ways to perform his music. Don't take all the blather too seriously and have fun exploring. If you're like me you'll probably end up with a small collection of equally enjoyable different takes on the music. Sometimes I like my Mahler dry, sometimes schmaltzy, sometimes bombastic. You'll eventually find the Mahler performances that suit you--there are plenty of really good ones out there.