The decision is of course a personal one based on your flying patterns and destinations, corporate contracts (if you primarily fly business), and similar questions.

Generally people, of course, tend to focus on a single carrier or two to accumulate miles in just a few programs and achieve elite status. Assuming you're in MKE, should you still fly on Midwest? Let's look at a couple key items with Midwest:

(a) Certainly Midwest will have fewer flights with Signature seats, and now an upgrade fee will be necessary for get the larger seat on flights where they have them. However most other airlines at MKE don't have premium seats on most (or any) of their MKE flights. And the two which do have premium seats...AirTran and Northwest...charge more to upgrade than Midwest does. Midwest will no longer offer a premium seat for free, but that change does not make them worse than the competition, does it?

(b) Midwest has definitely dropped some destinations, but they still fly nonstop year-round to far more destinations from MKE than anyone else.

(c) Midwest's agreement with Northwest means that you can still earn Midwest miles if you have to fly somewhere Midwest does not, or conversly, if you prefer to earn Northwest miles you can still fly Midwest when it is advantageous. The airline with the 2nd most nonstop destinations from Mitchell -- AirTran -- does not have the same ability. If you need to fly to Hartford or Birmingham or London, for example, you cannot earn within their program.

It's certainly a personal decision based on your own circumstances. If you've sworn off Northwest, for example, then the FF partnership is not helpful to you. But in general switching to other airlines means most destinations are connecting flights, and most flights (except NW and FL) are all-coach seating.