1) Bang for the buck is hard to quantify. Fares vary wildly. I've gotten cheap transcons on CO, but I've also gotten nice webfares on DL out of ATL.
2) Spacious in coach would have to be AA with MRTC. Spacious up front would have to be the flights that DL and CO operate domestically with intl configured aircraft. Nothing like a nice BizElite or BizFirst seat to relax into.
3) Since September 11, Continental has both coach and first class service locked in by a LONG way. I flew 2 transcons each on CO and DL on consecutive days last month (ATL-DEN in 757 F, PDX-CVG in 763 Y, EWR-SEA in 73G F and SEA-EWR on 738 F) and couldn't believe the difference in service levels.
4) 152 segments so far this year. 65 on NW/CO, 64 on DL, 8 on AI, 6 on AA, 3 on CX, 2 on HP, 2 on TW, 1 on UA, 1 on WN. I think thats it. May have forgotten some.