The effects suite included in Sonar was major factor in me buying it. When I was researching all my DAW options it became very clear that if I were to get anything else I would have to spend a fortune buying extra plugins (and synths/instruments). I didn't even know what most of the stuff was but did not want to get stuck as I progressed.
Sure enough almost everything I've ever had a need for as far as effects has been there in the Sonar package (and sometimes there will be three or more different plugs I can try out to do the same thing).
There are exceptions. Like we don't seem to have a straight up frequency generator (not really an effect but still an odd omission). There isn't a basic stereo field analyser (again not an effect). An in track tuner would be nice (instead of having to open an amp sim everytime or plugging into a hardware tuner). I'd really like some kind of dynamic EQ (there are workaround options but they're a little complex that hinge on the Cakewalk Analyst plug... which I don't think is included anymore). Some more mastering tools would be good too but understandably absent (because that stuff is expensive).
I'm almost positive that many of the included plugs aren't as good as others on the market but for the price and the fact they are simply right there (and tested to work with Sonar) I'm not left wanting... too much.
I seriously thought I would have had to spend WAY more on plugs over the years but I made a policy of first figuring out whether the tool already existed in Sonar before getting all GAS-ey. Almost never been disappointed and the things I can't find are either REALLY expensive or completely free from third party developers.
The only extras I've bought are upgrades to the Sonar versions of Guitar Rig and TH2, BFD Eco (this was before AD2 was included... still don't regret buying it though), CA2A, the Concrete Limiter... and I honestly can't think of much else aside from some sample/patch banks.
Things I think I need to buy would be some Izotope stuff (RX and Ozone), Melda Productions Spectral Dynamics plug and Overloud MarkII Bass.
I think I might like to get NI Komplete (or maybe just Kontact) so I have much more instrument/synth options for soundtrack type work but I get by pretty well with the included synths. Their sample/sound banks do leave a bit to be desired in regards to sound effects though.
Cheers.