Hey Hackett - sorry you're not really having much luck here - I also subscribe to the 'would never use one' idea.
Quick word of warning, although I'm sure you've done your research - you seem quite knowledgeable on the matter.
I got a phonecall a couple of months back from an old mate from school, who'd seen me talking horses on facebook. Neither of us had been interested in horse racing when we'd known each other, but we met up and had a pint. He was raving about these tipping services, about how they all show a good profit but since he'd joined they'd been terribly unlucky. He was putting quite a decent chunk of cash into these things and had been loosing significantly for about 6 months.
I looked them up after that conversation. An interesting fact is that the advertising these services put out in the likes of racing post is allowed to use figures which do not include all their selections. So for example, when they're doing well in, say, jumps, but poor on the flat, they just post jumps, or in ireland but not england, or whatever. The point is they don't have to declare if this is the case.
For this reason I would never go anywhere near one, even as, as Ron says 'an investment'.
I do see the attraction - especially if they explain their logic in detail, as a tool for learning the ropes, but I couldn't be having one as a long term plan.