Until recently I used GPO and Garritan Jazz & Big Band as samples to run in Sibelius 4 for the purposes of composing and arranging. At the time maybe 2005, JABB was the only reasonably priced sound library which had a tenor sax sound I could bear to listen to. In fact I'm pretty sure that the particular one I used was sampled from Iain Ballamy. It was however a bit of a faff to get the libraries running in Sibelius and took lots of enquiries and help from chat rooms etc (I wasn't very savvy re this stuff).
Following a hard drive failure and an upgrade to Windows 7, 64-bit and Sibelius 7, it proved impossible to get the original versions of GPO and JABB which I had to work with my new set up: However the samples that come with Sibelius 7 itself are greatly improved and for my purpose completely adequate. In fact the soprano sax sound is much better than the Garritan one I have.
So it sort of depends what you are trying to do I'd say: for me the latest vesrion of Sibelius as a standalone is great for composing and having something pretty realistic sounding to play back to yourself.