Oh boy... it's been a long time, but I've read quite a few Conan pastiches. I first got into Conan just over 10 years ago, when everything by Robert E. Howard was out of print and hard to find. I had seen the Arnold movies and I knew that the Conan character had quite a legcay, and I had to start somewhere. Here's a breakdown of the different Conan authors and their works which I've read, with some comments...
Steve Perry
Conan the Formidable - first Conan novel I ever read, a strange adventure story, and mostly crap
Robert Jordan
Conan the Destroyer - better than the Arnold film
Conan the Invincible
Conan the Defender
Conan the Unconquered
Conan the Victorious
Conan the Triumphant
All good reads, but lacking that special something that makes Robert E. Howard unique. Definitely oversexed, but at that time in my life, that was a plus

The recurring lust-hate relationship was Karela was OK, but nothing like the epic quality of Queen of the Black Coast.
John Maddox Roberts
Conan the Marauder
Conan the Rogue
Conan and the Manhunters
Conan the Bold
Conan and the Amazon
Good reads for the most part - I particularly liked the first three. All pastiche writers have their ups and downs, but overall John Maddox Roberts might have been the best of the bunch
Leonard Carpenter
Conan the Warlord
Conan the Hero
Conan the Gladiator
Ehhh... Hero was pretty good and featured de Camp's character Juma, but for the most part Carpenter's work is blah and at times hard to get through.
Roland Green
Conan the Relentless
Conan and the Death Lord of Thanza
HORRIBLE! Not only does Green fail to capture the essence of REH, he can't even write a good story. I couldn't even finish Thanza it was so bad!
Sean A. Moore
Conan the Hunter
Conan and the Grim Grey God
Good stories, good reads... but way too D&D-ish! Hunter has a priest of Mitra who functions exactly like a cleric, for Crom's sake! I liked them at the time, but having since read the work of REH I can see how flawed they were.
I'd like to point out that these are the pastiches I read BEFORE I ever got my hands on anything by Robert E. Howard. Then Baen printed a trade paperback series of Howard's non-Conan work, including a volume of Kull. I picked that up and read it and was like, damn, this is REALLY GOOD! Years later, my wish was granted when Del Rey finally published all of Howard's original Conan stories in three volumes. Needless to say, I snapped them up and loved every paragraph. Finally reading the REAL Conan felt like a homecoming. Also at that time, I got ahold of a few more pastiches, listed below...
Conan the Barbarian (de Camp) - I had to read the movie tie-in once and for all, and it was no better and no worse; I liked it
Conan and the Spider God (de Camp) - good, better than most pastiches, but not quite as good as Howard
Conan the Swordsman (de Camp, Carter, & Nyborg) - I liked this set of short stories, almost as much as I liked the originals by REH
Conan the Rebel (Poul Anderson) - disappointing! Not a bad read, but I thought it was going to be about Conan and Belit's adventures together but it wasn't... Belit only appears at the very beginning and the very end.
Conan and the Road of Kings (Wagner)
Conan and the Sword of Skelos (Offut)
These were decent pastiches.
Conan and the Emerald Lotus (John Hocking) - one of the better pastiches, somewhat closer to REH's style than Robert Jordan, John Maddox Roberts, or other recent pastiche writers
Conan of Venarium (Harry Turtledove) - NO COMMENT