Well, you asked for opinions, here is mine

- Guineapig clothing is a way of getting money out of people who are silly enough to part with it. They don't need clothes, as a previous poster said, because they are born with their furry clothes...they are already adapted for the environment unlike us humans, and the clothes would be purely for our amusement or to cater for some strange type of maternal instinct. Guineapigs are not dolls to be dressed up, and whilst I can imagine children wanting to dress them up, I can't think that an adult should be entertaining the idea ?!
What worries me also about clothing for animals is that it underlines a trend in treating animals in the same way as humans; our pets do not have the same needs, the same behaviours or desires as we do, but by treating them as if they do have human needs people entirely misunderstand the animal. I've seen recently a DOG BUGGY ! designed to take the canine companion out in the manner of a baby. I saw someone using one and really wanted to say - THAT is a DOG, dogs run and get their fur and paws dirty, you are being cruel to it by not allowing it to walk and run and exhibit a dog's normal behaviour. Surely our pets have a right to be treated in way that is most beneficial and natural to them, and showing an understanding of their true guineapig requirements (rather than a perceived need for clothing!) would do less damage to them.