Pros
Interesting work, smart, interesting people to work with. A chance to contribute to new medicines and improve healthcare. Good pay and benefits. The company is socially responsible both in terms of charity, and in terms of contributing to improved healthcare in the developing world.
Cons
A rolling reorganization has been running for 3 years now. Recent business setbacks have increased pressure on budgets even further. Some groups are in their 3rd or 4th round. Poor morale in those who are left following the preceeding series of cuts. No obvious plan for improving research output, but plenty of plans for cutting costs. R&D appears to be run by MBA's at this point. Currently GSK has the stability and security of a small company with the bureaucracy of a large company. Job advancement is limited by the fact that the company is contracting, but losing employees faster than managers.