I was once op-upgraded from Business to First on AA from ORD to LHR. It was a bit roomier and I really liked the little cabinets down near the floor for your stuff, so you had more things near you during the flight (books, laptop, etc.) I can't remember but the bed may have been lie-flat, which it is not in AA's Business Class. What was most noticeable to me was that the food service was more extended- started earlier, extra courses, etc. In a way that wasn't a great benefit to me- I do like to try and sleep part of the flight although I rarely succeed. On this trip I was heading straight to the office when I landed. Certainly I could have had the option of getting the whole meal served at once, but I figured (correctly) I was unlikely to fly TATL First again and may as well see what the whole experience was like.

Conclusion: while there are acres of difference between Coach and Business TATL, the difference between Business and First was not enough for me to pay the extra, in miles or dollars.