In full agreement with Cyparski and Pahtcub.
I used to mix on an old (but good spec) Sony DTS hifi system. w/sub. My mixes usually came out a little harsh. After some time, I bought Edirol MA-15Ds, which gave a very flattering sound, but over-smooth, and with not much detail. My mixes sounded weird, then slowly improved as I adjusted a few settings and compensated for what I identified early on to be a strange 'hole' in the woofer/tweeter crossover.
Then I bought a set of Yamaha HS50ms. Beautifully detailed, tight, focused bass within its range (for a 5" cone) and great imaging... but never felt I could entirely trust them for EQ purposes. They were so detailed that my mixes ended up sounded muddy and had a lack of detail.
Finally got a set of Adam A7x monitors which sounded a lot closer to the Edirols in terms of character, rather than having big differences in the sound, I can hear many smaller differences.
I haven't done much on the Adams yet, but can say that in the end all my best stuff was done on the Edirols because I had learned to pull out more detail in the mids and hi-mids. To tail back the lo-mids a touch and a few other bits. My desk is set into a bay window area and my room has no treatment.
I'm planning to sell my Yamahas and some other bits and buy a pair of HD650 headphones. My room should be treated better, bit if it's not; the headphones take my control room acoustics out of the equation and I have a reliable monitoring solution that I can also take with me
(I did really like the NS-10-like white woofers though)
If you can't pitch too high money-wise, but do want something reliable and good, I'd say the Sennheiser HD650 phones. There is a newer and more costly version, but I don't think the price difference will justify the benefits. I can safely say, new monitors never improved my mixes. I had to do that through learning whichever speakers I was using at the time and comparing on two earphones, two headphones, my monitors, desktop speakers and my car. Once I've spent a little time with all those, then re-checked on my monitors, I understand their shortfalls and know how stuff should be sounding on my monitors to sound right...