RAR is arguably the best 'megadungeon' ever published. Quite definately one of top ten products published in the past 10 years.
As regards the rest of NG's proct list, production quality is very high, but writing quality, cartography, and illustration all vary quite widely. The early Rappan Athuk 1-3 have the same high quality writing, but the mapwork is very poor. Same with the reprints of the Maze of Zayene books. In both cases, the writers are very good, but NG didn't have a desent quality cartographer on board. The maps look like badly drawn maps on A3 that have then been reduced on the office photocopier.
On the other hand, The later works, RAR, Abyshor, Glades of Death, Mesopotamia, etc, all have high quality graphics, high quality mapwork, and good writers.
And then you have the likes of Dead Mans Chest, Morrick Mansion, etc, which are all pretty much run of the mill d20/3E quality, and it's down to individuals if they're worth buying or not, and the same goes for the supplements like Demons and Devils, Vampires and Liches, etc. And Grimtooth is a direct badly scanned photocopy of the original, and you can even see the photocopy marks. It's an absolute piece of sh!t.
In a nutshell, stick to buying what NG wrote temselves, and avoid what they reprinted/rehashed for others. On the whole you'll avoid 90% of the crap stuff that way. Those things should be left for the completists.
Not buying a box of 12 RARs was my biggest ever mistake in running my eBay business. I had the chance in 2005/6 to buy a box lead when my local games store bought a case, and I didn't buy a single copy on account of them refusing to allow me to open the
SW on one. At that time they were £50 a set. One sold on eBay a week or so ago for £135.
This week I've been mostly eating . . . The white ones with the little red flecks in them.