Well I've just bought my first SACD (only had the AX5i/757Ai about a fortnight now) and on this limited basis I'm quite encouraged.
The disc in question is 'Comfort In Sound' by Feeder. Hardly an audiophile recording, but a real-world test of SACD IMHO. Pleasingly, the disc is a hybrid affair with stereo CD and SACD tracks plus a multichannel SACD mix too.
The area of largest improvement is in soundstaging and treble detail. The music in question frequent has distorted guitars/bass and can sound a bit 'mushy' on regular CD. However, both SACD tracks demonstrated improved placement of instruments with the multichannel mix being superior. The multichannel mix is well done incidentally, with the other channels merely adding some extra ambience and depth rather than silly instruments appearing behind you.
A word of praise for the AX5i though, it's audio scaler feature claims to improve 16bit resolution to 24bit and in fact the comparison of CD with 'hi-bit' on to stereo SACD is remarkably close.
The AX5i can be set to SACD direct which I believe takes the raw DSD stream from the player or have it turned off which utilizes the amps bass management and MCACC features. Personally I preferred direct off.
Still early days, but I think I'll be investigating SACD further when (eventually) more recordings I like appear.