I recently traveled in paid business on UA on a flight that I knew had First Suites... it was too much, I couldn't resist. I had to spend the 25K (it was from asia to the US) for an upgrade from business to first 10 hour flight. My other thought was: "who knows when I will get to fly international first again? Live for the moment".

The verdict: nicer champagne, nicer glasses, nice flat bed, somewhat better food. Not worth 25K miles over business. I heartily agree with the other posters that say the difference between coach and business is many times the difference between business and coach (despite the difference in miles it takes to upgrade between the two, due to the number of seats in each). I really have plenty of room in most international business seats, and I have trouble sleeping on most flights for more than an hour or so, so the difference in seat (as long as the business seat has a footrest), isn't really going to enhance my sleeping ability. Both seats have EmPower jacks on UA. In business on some carriers (such as SQ), the seat is motorized as well, so you may get close to the same comfort as you would, if you don't mind limited recline.
Yes, you get more aggressive service in first (people doting a bit more), but I have had just as aggressive of service in business. And as for the food - first of all, it's a short part of your triep. But it's really not that much better than the already premium stuff they serve in business. You're eating on china with cloth napkins either way.