Underrated
Talcum powder, clear transition boxes (I like them), neoprene hats, speedo cheapo goggles, DA C24 wheels, OMM 10L ruckies, Nuun, little gel bottles, ceramic bearings, specialized cycle shoes, leather gloves/saddles, Richmond Park once you "get" it.
Overrated
the posh foods, Frostie bars and Asda own are just as good, gels not in little bottles, Aero bottles, full mask "tri" goggles, Schwalbe Ultremo tyres, great but they rot in 6 months, shit track pumps, specialized saddles (ouch), up their own arse LBS's