I think it is useful to check out the comparison chart, figure out where you fly and what benefits you want.

All the airlines have issues. AC was once a very good airline and still is on some of its routes.

ML is correct about reward availability for less than SuperElite though. Also you won't get as many miles as I did without hitting 100k miles (Super Elite qualification)--you get most of your bonus miles at 50K and above. It works for some of us but not for others . . .

UA is still the worst of the bunch in status and mileage earning but if you fly out of the USA or in the USA a lot, you might find a few promos that suit your patterns and might increase your mileage to AC like levels. Remember you only get the 100% bonus for PremEx and 1K on LH and UA flights with UA's Mileage Plus, not with the Star Alliance crew. On AC and LH you get your bonuses on all Star carriers status miles. If you fly a lot in business or first, then LH is definitely worth a look, especially with its 2+ year validity each time you make top tier!

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BlondeBomber's Star Lounge Guide
BlondeBomber's Star Alliance Gold Comparison Chart 2003
BlondeBomber's Hilton HHonors Rewards Hotel Levels (1 MB!)
BlondeBomber's Starwood Hotel Levels