Cons: nothing excels
These are ordinary performers, i had them and although they sound reasonably good on their own, that only applies when you have nothing better to compare them to. If you know what your looking for there are better headphones out there which are also cheaper and sound much better, like the Sennheiser 202's or 205's, both excellent sounding headphones, the 205's are the same price around $100 and the 202's are nearly half that, but the 202's are my favourite.
The bass on the K99's is lacking slightly, the midrange is a tad recessed and the treble is a "bit" wonky, probably not the worst headphone you could buy, but if there are no other options then they will do the job nicely. I wouldn't call them awful at all, just that they don't excel. However, if you amp them they do noticeably increase in quality but still i wouldn't call them the best, you know what i mean.