The Thieves' World books are good. For them to be great, they would have to have been OGL rather than d20. I think it really deserved it's own magic system and spells, but I can understand the market advantages of maintaining compatibility with d20. They were trying to walk the line between staying true to the content and style of the novels and utility to the widest possible d20 audience. I think they succeeded.
From a Conan perspective, it can be used to enhance your game if you wish to encompass other sword-n-sorcery styles than REH. If you want your campaign to be similar in theme, tone, and style to REH's Conan, then Mongoose's treatment does the job extremely well as is.
On the other hand, if you want to be able to have viable non-martial or non-sorcerous PC classes, TW is an excellent addition. Commoner just doesn't cut it. TW has the Survivor (the common man who's just too tough to die) and the Savant (think a viable PC Expert or non-sorcerous Scholar). The Assassin is also a decent class although the archetype is easily done with Conan multi-classing. If you want sorcery that isn't quite as dark as portrayed in Conan - TW is a nice middle ground between Conan style sorcery and D&D's magic-as-technology/skills. You have Mage, Witch, and Priest as your primary spellcaster paths - each has unique abilities and spellcasting strengths and weaknesses. Thematically it leans more towards Conan but utilizes the D&D/d20 spells so you can pull from a lot of WotC and 3rd-party source material.
I quickly converted to the Conan rules once I picked them up. Neither I nor my players wanted to scrap our campaign, however. I use the Conan RPG as the baseline rules set for my Kingdoms of Kalamar campaign. I incorporated elements from Thieves' World to fill in the gaps in my campaign. I always liked TW 3-sources of sorcery: arcane magery, witchcraft, and priestly magic. As much as I like the Conan sorcery system for Hyboria, TW's treatment fit better into my Kalamar campaign. I calculated Conan Parry and Dodge values for the TW classes I was going to include and it's worked like a charm.
Also, TW treatment of races is more easily emulated for various campaign settings. Conan's rules obviously use the Hyborian races. I found it difficult to develop racial abilities for the races in my campaign that were on par with the Hyborian ones in the Conan RPG. By using the TW treatment of Cultural and Background mechanics, I achieved the balance I wanted without resorting to D&D Player's Handbook blandness.
So as a complementary source to Conan or a grittier alternative to D&D, TW fits the bill. It does not, however, compete with the Conan RPG.
As a final comparison point, by using the Conan RPG and supplementing with Thieves' World, I can pretty much come up with a character write up of the large bulk of characers from fantasy fiction or film that emulates the source material pretty well. Something that was impossible to do using straight D&D rules.
Azgulor
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