I have a tri bike w/Ultegra from 2001. It had a 9 speed 11-23 Shimano cassette. Wanting a little more low end for a very hilly race I'll be doing, I bought a 9 speed SRAM 11-26 (AM PG 950) cassette as folks on this board said they'd be compatible.
So...after installing, yes it all works but it's rattly and noisey. The sound seems to be coming from where the chain leaves the cassette. It's also a little rougher shifting and jumps a bit.
The 11-23 Shimano was very quiet and smooth.