Ha! I’ve just spotted Bill Gates is wearing my watch! It’s not a F-91W, it’s a GW-M5600:
http://www.casio.co.uk/products/timepieces/g-shock/origin/product/?v=GW-M5600-1ER
(Note extra button on top right of watch – i.e. two silver buttons facing Gates’ hand)
It’s great, solar powered so hasn’t needed a battery for the 8ish years I’ve had it, waterproof, shockproof, keeps perfect time because of the radio sync thing, has the glowy backlight if you need it. I wear it practically all the time, which is a bit of a shame because (yeah OK, haters gonna hate etc) I’ve got a lovely Tag that I inherited from my father-in-law.
To actually contribute something useful to the OP – don’t take the Omega to the “recommended” Omega guys in Burlington Arcade, London. They were shambolic and totally cocked up my mum’s vintage (well, old but in good nick) watch, lost one of the arms, fitted the strap back on back to front. And kept her waiting for literally months and charged hundreds for the privilege!