Much like Tip Of The Tongue, Spore is a decent story that doesn't feel like a Doctor Who story. Part of this may be due to the fact that The Eighth Doctor has always been the hardest incarnation to pin down in a written form, having had the smallest amount of screen time and the most erratic portrayal in the books, comics and audio plays. As such, The Doctor in this story seems rather muted and there is little to distinguish him from any other incarnation save for a distinct lack of the other Doctors' usual tics.
The other problem with this story is that there is very little sense of drama. The Doctor himself is immune to the alien menace in question, so there is no worry of him being imperiled. His one-shot companion in this story - Captain Evelyn Chan - is a non-entity who serves no purpose other than to give The Doctor someone to talk to besides the spore itself. Even the spore itself - which will doubtlessly remind comics fans of the Venom symbiote from Spider-Man - seems rather generic as far as alien menaces go.
I'm hard pressed to think of anyone but the most devout of Whovians who might enjoy this story. It isn't bad but neither is it truly good. It's just sort of... there.