Is giffgaff viable?
Well, yes, but no more so than any other of the smaller networks. It's pretty well established, apparently profitable and customer numbers are growing year on year albeit fairly slowly.
I think the key point concerns giffgaff's USP.
My view is that it doesn't really have one. Or to put it another way, it has the same selling point (note the lack of the word 'unique') that every other similar small network has; that being that its product range appeals to a certain section of the market.
giffgaff does have some differences compared with others; the lack of traditional support for example but suggesting that the community is a selling point is a decidedly moot point. For those who feel that good support is important to them, I'd suggest would find giffgaff's methods more likely to be off putting than positive.
A good question could be; does giffgaff, or any other network for that matter, really need a USP?
I think not. All that's needed are products that appeal to the market.
For giffgaff to remain profitable and continue growing in the future, all that is needed is to continue providing a product range that is appealing to a section of the market. giffgaff will never rival any of the mainstream networks but, if giffgaff's management and marketing continue in the right way there's no reason why it can't continue providing a service for many years to come.