Last year I flew on United's 747-200 from Los Angeles to Honolulu and it was aweful! I'm sure many people who have flown on UA's 747s would agree that their economy class seats are cramped and uncomfortable. In addition, I found that the air conditioning system was not very effective since it got quite hot in the main cabin. On the way home, I asked to sit in the last row window seat in the main cabin and it was much better since there are only two seats instead of three, you're closer to the lavatories and you can get up and stretch (there's room behind you). The Pratt & Whitney engines were not the quietest I've experienced, but they do the job. In conclusion, if you are sitting in economy on UA's 747-200, try your hardest to get the last row window seat... Lie if you have to (I did... I said I was clausterphobic) and you will feel a little better than the other passengers.
Hope this helps.