There's a useful book that I think is called Canon EF Lenswork III, useful because it contains MTF charts for most of Canon's lens range together with comparison MTF charts with both the x1.4 and x2.0 extenders. Its only flaw is that there's a couple of unfortunate errors that can trip up the unwary.
It clearly shows that image degradation with extenders is far from uniform across all lenses. This shouldn't come as a surprise, lens manufacturers have long talked about extenders being optimised for specific lenses, but looking at these MTF charts shows just how marked the effect is. One example, and this is from memory so don't take this as gospel, was the 70-200 2.8 in the IS and non-IS versions. The IS version has the performance edge without an extender, but with either the 1.4 or 2.0 extenders then the advantage goes to the non-IS version.
I think that your style of composition will also influence your satisfaction. Quality with extenders seems to fall away more quickly as you move from the centre to the edges (which would tend to support the previous post about preferring primes to zooms with extenders), so if you usually compose centrally then you're more likely to find extenders worthwhile, conversely if you often place critical detail out towards the edges then an extender may prove a false economy.
