http://www.cakewalk.com/products/SONAR/X1-Producer/feature.aspx/Instruments-in-X1-Producer cakewalk soundcenter is actually a nice little jewel for "on the road" type stuff and there are even some expansion packs that might not be too bad if you need them.
FWIW - I have never used Session Drummer 3. I did use Session Drummer 2 on one project, but I could have just as easily used EZD on it. I have EZD and Superior Drummer and so I never use Session Drummer. Session Drummer 3 is not "better" than EZD, if you want "better than EZD" then crossgrade to Superior Drummer 2. it's killer.
TruePianos are meh. IMO. at least the module(s) that come with Sonar. some say that the ones you have to buy separately are better, but I use Kontakt Classic pianos myself.
GR4 comes with producer only, I think. it's pretty good, but the LE version is not really anything spectacular if you already have GR3 full version. the only thing that GR4 will get you, however, is compatibility with 64bit. GR3 won't even run in 64bit sonar with bitbridge, but GR4 is 64bit native.
http://www.cakewalk.com/products/SONAR/X1-Producer/feature.aspx/Effects-in-X1-Producer I really like the PX64 and VX64. they are really good plugins, IMO. I also like the tube leveler, but it's difficult to use without "overusing" it if you know what I mean.
ProChannel is quite good, IMO, except for the tube saturation, which is just "OK" - but some people are having problems with it behaving bad. I haven't seen the problems they have, however, so it's hard to say if you will have those problems or not.
Perfect Space convo reverb is actually quite good, too, IMO. I was using it exclusively until I recently bought IK Multimedia's Classic Reverb.
Producer is certainly worth it if you need some of the tools above.