Your right that VX-478 does not appear to stand above the other PI's in terms of efficacy and resistance. We'd still be above 40 if it did.
There was worry about the skin rash issue which drove the stock back into the high 20's. That issue seemed to be resolved with the Phase II data at the last HIV conference.
There are some factors which may win market share for VX-478 with time:
1) Glaxo is the #1 marketer of anti-HIV therapy.
2) VX-478 is given as a twice a day regimen as compared to 3 times a day for other drugs.
3) It is the only HIV drug I know of being tested for children. I don't think they've released any data.
4) There was some indication in prior trials that VX-478 crosses the blood brain barrier. If this is the case, VX-478 would be an important cocktail component, especially in patients with active viral replication.
5) It's being tested in various Cocktail combos, which means clinicians will know that it works with other drugs rather than have to find out empirically. (Eg: Glaxo is testing it with Abacavir, a new antiviral)
6) Clinical Study safety/tolerance data I don't think shed much light on what its like for a patient to be on the drug. The true test of VX-478 will be when patients start taking it and it builds a reputation (either good or bad with clinicians and patients).