Pros

Elite members get a redeemable earning bonus on some airlines in higher cost ticket buckets (from 2012 this covers almost all ticket buckets on UA/CO). On United redemptions you do have the option of increasing the miles you pay by 100-125% and virtually guaranteeing redemption availability. Elites are exempt from baggage fees. There is a mileage earning credit card (Amex) available to UK members.

Cons

Redeemable mileage earning is lower than BD (typically 150% for business). Only Elite members earn minimum 500 miles per flight. Number of non-earning ticket categories within Europe and within N. America is higher than with BD (although the number of intermediate earning - 25%, 50% - is fewer). Saver redemptions (i.e. normal redemptions with restricted availability) require about 60-70% more miles than BD for business, economy is marginally more (in effect therefore the number of C class segments flown on most airlines other than UA to earn a C-class redemption is roughly double BD). UA*G requires 50k miles (although this is a Con, there isn't really anything better than this except A3 and BD of course). UA*G does not provide lounge access in US unless flying internationally the same day.