It depends on many factors - flying in Y or E+ in the main cabin, I assumed
And, the specific aircraft & seat design - sometimes, you need a good seat cushion if the "padding" are thin, light & lack lumbar support. Newer seats have built-in adjustable headset/support, so an U-shaped neck pillow might get into way & make it worst.

Here's one that can go from U-shape to neck/lumbar use in 5 seconds - http://www.amazon.com/Lewis-N-Clark-...+travel+pillow
We have a similar one and the micro-beads are very firm & supportive, and mold itself to support the neck's tilting/leaning against the window. (Extra pillow provided by the FA sometimes help as extra ... we carry soft flannel pillow cases to "stuff" them inside first)

Mrs. L also take along a travel-sized memory foam (extra firm) pillow for her lower back support, and also use it to "pop" up her iPad2 to watch video or play games, etc.

Here's another one - new design - looks good & worth trying out for its claims http://www.amazon.com/Pillow-British...B8ADR0NFCFRM5B

Ours have clips/straps so they are attached to the carry-on/backpack while boarding the aircraft.