I predominantly purchase WoTC books. I limit my campaign to WotC books though for several reasons:
- I trust that in terms of balance they are at least within a tolerable range. Further if there is an issue, because their products are well known and discussed on many forums, any cracks quickly appear and can be fixed just as quickly.
- These products use a lot of the information/rules that I already have access to - the complete series & other supplements etc. 3rd party products pretty much only have the SRD and Open Content.
- The high quality of production (full colour/suitable/evocative artwork - the recent "Drow of the Underdark" has raised this quality up another notch in my opinion).
- It's easier to limit resources in a game - Core Only/ Core and Complete/ WotC 3.5 Catalogue
- It has the "official" tag on it. The campaign I am currently running has zero house rules. All rules and options are taken out of WotC books. I have found the rules arguments that seem to develop in other games that I have played in, simply do not happen in my game. Everyone knows the "official" baseline and works within that. Perhaps it's just that most people (at least that I know) seem happier to play "official" rules rather than "fruitier" DM house rules where perhaps not all agree/appreciate/like such changes from the "official" guidelines. YMMV on this point - this is just my personal experience.
I also have a reasonable-sized collection of non-WotC material. Most of this is purely for interest with almost none of it seeing the light of day in my campaign. Some material, I bought because of quality/collectibility - Ptolus, George R R Martins ice & fire d20 collaboration. Others I have bought for campaign construction assistance - Everything by Expeditious Retreat Press, a variety of peculiar pdfs. Others I have bought because I simply wanted them - A lot of Malhavoc stuff, Book of the Righteous etc.
And then unfortunately, I have purchased non-official materials that I regret (but would prefer not to mention).
I'll give 3rd party stuff a look at (and if it's good I'll purchase it), but for the moment, none of it will see active use in my campaign.
Best Regards
Herremann the Wise