We have redeemed a TON of UA miles in past year, and have NEVER had a problem getting what we wanted on a bunch of Star partners, on a bunch of routes to a bunch of destinations in all classes.
I dread dealing with KLM for partner awards - they want you to FAX or MAIL your "requests in" - even on the verge of 2004.
I hear anecdotally that getting an It'l Biz/First award on CO is all but impossible for most routes, most times of year, so we moved all our CO miles into Hilton, as did many others.
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~ Glen ~ sipping bubbly from UA 747-400 exit row 15A near you SOON!