Since the US tends to be my most frequent destination outside Europe, I've been concentrating on UA for the last couple of years, and am about to make 1K for the second time. For 1K level it's a pretty good bet particularly if you fly mostly economy and to North America a lot. Below are the main points from my point of view -- lots more one could say:

Pros: 100% earning on almost everything; fairly low *G qualification (50k miles); very wide network now that joining with CO. 1k level is attractive with 6 system wide upgrades (also usable on single LH segment on day of travel if you get a paper certification 14 days in advance). Low taxes/fees on redemptions (e.g. z3-z1 o/w Y is 30k miles + £1.50) . Extra availability for elites. E+ on UA flights (extra legroom). Unlimited domestic upgrades (but don't hold out much hope if you are lower level gold). C+M sometimes available.

Cons: The lower *G levels will have reduced bonus miles from next year -- only 1K level still gives 100% bonus (only on certain airlines though). No domestic lounge access (unless same day international flight). International mileage upgrades on UA require co-pay except for YB. Class of service bonus not as good as DC (varies by booking class and airline). No soft landings - i.e. no gold->silver even if don't fly at all. Now requires minimum 4 UA/CO segments p.a.