I've given up on splatbooks. A year or two ago I gave away a mountain of WW stuff at the swap table at Recess. I realized that all I really needed to run Vamprei was the core rulebook. Even the last time I ran the game over a decade ago, I had dumped all the "canon" NPCs and setting material. Hell, I even dumped all the bloodlines. None of it was necessary to the type of game I wanted to run.

-- Ben

Depends on the system.

I like Ars Magica's splat books because they add a huge amount of detail to the game. Tribunals have thousands of years of real world history and legends to draw on and the Realm books help open up a lot on Magic, the Infernal, Faerie, etc.

I like add-on books with crunch, as 90% of the time, the fluff doesn't apply to the campaign I'm in/running, but I can adapt the crunch to fit wherever I want.

YMMV.

Okay, I'm'a gonna break this down a bit:

D&D 2nd Edition:

The Complete Series, which was mainly character-creation rules with a bit of fluff about 'day in the life of" stuff--it was actually useful, but the DM-oriented splats (like the castle book) left me kinda cold.

D&D, 3.X:

Well, there was the way they undermined the first round of splatbooks with the new release. I'd've rather had a straightforward 3.5 errata than a whole new series. Still, for D&D, I found the early splats for both 3.0 and 3.5 to be fairly useful. Setting-specific splats appealed less, because I rarely played in the official settings. Later splats I didn't bother with at all--it just got to be too much.

oWoD:

Ah, where splats became a real and permanent thing in this hobby. In general, I liked 'em, though if you were playing more than one or two of the lines, prices got pretty painful after awhile. Still, they had a LOT of setting material and background info, as well as some decent mechanical bits (Mage had less mechanics, simply because the core mechanic of the Spheres was, honestly, all you really needed to do anything you can think of--that was the point).

TORG:

The first splats, which basically rounded out each of the realms that had invaded Earth, were awesome. Then they reached the end of that money-train, and decided to run right off the rails. First were a few books dedicated to other realms, that mostly entailed a lot of power-creep; then came the very low-quality gear books and other strangeness, which added very little value to the Possibility Wars setting as a whole.