Whenever I can get a Spider out, I find that they're fantastic and work terrifically well with the druid. My issue is that as they are not a plant, I normally have a tough time finding a moment to cast the Spider. The other issue is that he does take up 4 spellpoints. Now, that may not seem like much at all, but the Druid has access to many fantastic level 1 spells and you're giving up a lot of redundancy with it.
End of the day, I've had fair success and it does synergize quite well, but I'm thinkn' that I'll stop using it.