The 15,000 for "business" upgrades are actually from business to first class. Not many flights within North America have both business and first class cabins; most have just economy and first - which, ironically, is not as nice as international business class. Note that there is no co-pay for the business-to-first 15k mile upgrade, but there usually is for the economy-to-first 5,000 mile upgrade. See http://content.united.com/ual/asset/..._052010_v2.pdf.

As for the best value for miles: that really depends on what you value. Do you just want to keep as much money as possible in your pocket? Then wait till you have 25k points and get a domestic economy award ticket. Do you want to experience premium cabins? Then the best thing would to be save up for an international business or first class award ticket (or upgrade - but the copays can get pretty steep for cheapo economy fares).

It might seem like this would take a long time, but there are lots of hints you can glean from FT and even the United web site about gaining miles faster. For instance, UA has several credit cards that allow you to accumulate miles.

As for this question: "are the seats available for rewards so limited that International is difficult without lots of flexibility" - that depends on the destination, how many people you're trying to obtain seats for, the season, how far in advance you start looking, and which other Star Alliance partner airlines you're willing to fly. I'd say most people (at least FTers!) are able to find UA or partner international award flights that fit their needs one way or the other, but don't expect it to be a complete walk in the park. Note that upgrade seats are also capacity-controlled, but there are ways to hunt for them before you buy a ticket. Keep browsing this forum and you will learn them! (Or do a search.)