I hope count as someone who knows, lol.
I regularly use 1 in my 1K army, and anything over that always includes at least 2. Money wasn't really an issue as I bought the army deal anyway.
I personally think they rock, plain and simple. Their ability to create Scarab's helps them soak up Heavy Weapons fire like Lascannons without damage, if they're firing more, they must really hate that 55 pt. model. In HTH he's invaluable, providing you with a very nasty speed bump. Very few squads can get through T6, and the fact that he's fearless means they stay right there, waiting for a countercharge or just to be ignored. His ability to raise dead Necron's is icing on the cake. Spyder's are definitely worth it in my book. Use them to bolster your lines, and provide HTH deterents. You'd be amazed how many people hate them.
As a side note I'd recommend Duel Claws, rather than a Particle Projector. My Spyder's tend to get into HTH rather quickly one way or another, and that one round of (generally poor ) shooting you get off generally doesn't pay for the extra attacks you'll lose.
Xhadow