There is a basic flaw with the piano sample/waveform. The pitch changes as you hold a note but its not too noticeable in a mix. Strings are OK, but I added the Synth Wave I expansion card to my SY55 which gave me a broader pallete for string sounds.
I particularly liked the Voyager and Itopia patches when it came to choir sounds. They were the more breathy-synth type and not your realistic church choir type sounds.
Horns aren't bad either.
For $79 I'd take it. I mentioned b4 that my setup 10 years ago was a SY55 and DS55 (baby brother to the YS200). They complemented each other well.