I think it's safe to say that when compared to current multi-layer,single instrument libraries it definitely falls short of the mark, or even more expensive multi-istrument libraries currently used that cost significantly more than the either purchased for $49 or thrown in as a gimmie with Sonar as Studio Instrument currently is.
But for a product, that has not seen much development cycles since it was first introduced as a $49.00 muli-instrument library back with what, Sonar 6 (I could be wrong, I just know that I purchased it in 2007 when I was using Home Studio) it still works pretty well for fleshing out ideas quickly or for those without the resources to buy more dedicated or expansive libraries (which tend to come with a more hefty cost) to add parts to projects one is working on easily with usable results - it's really not that bad of a product. Back when I first got it, I used it quite a bit. Sure, now it's been replaced with Kontakt libraries for almost everything (strings, bass, drums, ...), Toontrack products for drums, Garritan GPO and Miroslav for orchestral, TruePianos for, well ... pianos, and a large variety of other products. I've come a long way needless to say.
But all in all, Studio Instruments is a pretty good deal for those dipping their toes or looking to quickly flesh out ideas or in support of other products, I just think Cakewalk never saw it as the go to product and at $49 I probably would not expect it to replace more expensive alternatives.
Just my thoughts on it - as always individual mileage may vary.