>I haven't heard any of his recordings myself. How do they compare to other conductors? Are they worthwhile?
I guess that depends on what you are looking for in this music. It seems like us Mahler "nutjobs"
can't get enough of hearing alternative views of what he was trying to say, so in that regard, I would say yes, it is always worth hearing another angle on this music. Now having said that, this set is quite pricey $$$$, so try and shop it at a discount if possible. I got mine from an Amazon vendor used for about $70 or so, which is a great price for this set.
As for the actual music, Gielen is not overly sentimental, but I do hear a lot of passion. He is amazing at points and great-to-good at others. If you dig the modernist aspects of the scores, you will probably find a lot to like here. If you go for the bombast, look elsewhere. It's not a perfect cycle (what is?) but there are moments and entire movements that are exemplary. The finale of M2 is a tad underwhelming for instance, but his M6 finale is as good as it gets to my ears. There is a TON of music here, so don't expect miracles at every corner, but overall it is a great sounding (albeit to varying degrees) set that is very consistent. And no worries on the SWF Sinfonieorchester Baden-Baden and Freiburg (say that 20 times fast...) - they can play their asses off and then some.
I personally wouldn't want to live without it.