Pros
Multi-disciplinary, so there's usually someone with the answer to your question. 9 day fortnight + flexible working Not a bad pension contribution Good career opportunities if you're prepared to blow your own trumpet... don't expect others to see it.
Cons
Process driven... so lots of forms and paperwork. Limited authority: there's massive restrictions on who can authorise purchases, training etc. which means there's about 20 people needed to sign something off. Lot of deadweight... company is so vast they slip under the radar, even with the draconian focus on billability. Visible time wasting on Facebook, BBC Sport etc. in the mornings and at lunchtimes. Role titles are there to fit with the payscale, not the actual job. There's hundreds of directors who aren't actually directing anything... they're just "do-ers" to meet billability targets. Long hours, and they never say NO to a client or pushback on questionable decision-making... they take the money and just keep saying YES.