Fascinating thread. My two penn'orth FWIW.
Firstly, as far as I can see On-One honoured the terms of their warranty. I have seen many threads about big manufacturers ducking their responsibilities by using obscure clauses in Ts&Cs to avoid payment/replacement. Granted there seems to have been some kind of recurrent issue with regard to the OP, but given that (to my knowledge) this issue hasn't been replicated with a large number of these frames, it's probably safe to assume that it is a relatively isolated occurrence and may well be related to use, or to the conditions in which the bike is used (it's pretty cold in Finland I believe).
Now I'm no On-One or Planet-X fanboy. I've had my issues with a wheel I tried to buy and I can second the earlier comment about threadbare socks, but a warranty is a warranty and I can pretty much guarantee that none of the "big" manufacturers will go outside of the terms of their warranties. They may offer a gesture, but On-One already did that. For people to suggest that they have been in some way out of order in terms of how they have acted is naivety. They're a business for God's sake. They have to make money. Just because it is mountain biking that we all love and hold dear, it doesn't mean that the companies that supply product should put all business principles to one side. If they did, we would have far fewer suppliers to purchase things from. At the end of the day a warranty is a warranty. If the customer isn't happy with the terms of the warranty then he shouldn't have bought the product in the first place. If he didn't read the Ts&Cs then he should consider that a lesson learned.
With regard to what he paid for the frame, there seem to be a number of comments along the lines of "If it was a £250 frame fine, but this was an £800 one. Different!" Well, it obviously cost more than a £250 456 frame, but it's not a fair comparison. The OP bought a very lightweight XC race frame for £800. Fine that's a big chunk of cash, but the equivalent frame from one of the big guys (or indeed Pivot) would cost anywhere between £1,600 and £2,500. So he made a decision to buy a cheaper alternative to save money. But there is a reason it was cheaper isn't there? If he had wanted the "big manufacturer" guarantee of quality (if indeed, such a thing exists), then I guess he should have shelled out from the outset. To reiterate this frame was significantly cheaper than its contemporary rivals. He knew this and decided to take the risk.
With regard to Brant's comment about "Wasting any more time", that was certainly counter-productive and in fairness he seems to have wised up to that fairly quickly. However, there aren't that many manufacturers out there who have people actively looking to address and resolve customer issues that are aired on social media. I think they should get credit for at least trying to be abreast of the conversations and dealing with issues when they arise.
Finally our "Not fit for purpose" friend. Well, a balanced view on that would possibly be that this particular frame was probably not fit for purpose in the environment within which it was being used. To extrapolate that to say that the company's product range is not fit for purpose and that their QC is rubbish is unfair and quite possibly unwise.
All in all I think that On-One haven't done a huge amount wrong here, though Brant may want to reflect on how flippant remarks may sound to other customers (and potential customers) who may well feel that the definition of customer service is doing all that is fair and reasonable to resolve an issue to a customer's satisfaction, regardless of whether or not they are likely to be a future customer.
Finally I'd be interested to hear from the OP exactly what he wanted from On-One that he feels would have been a satisfactory response to his issue.
EDIT - I've just read this back and it is - if I say so myself - a surprisingly balanced and well reasoned comment. Both for me and for this thread. For that I can only apologise and assure the STW collective that it won't happen again