The deal with the Spyder 4 is that if you get a solid unit it's a great tool for the investment and a great way to get into DIY calibration. I had excellent results with a Spyder 4, checked against an i1Pro & i1D3.
The Spyder colorimeters have a bad reputation due to the previous generations being very hit and miss regarding accuracy. They have improved each generation (just as most other companies have done) and as such you have a good chance of getting a fairly accurate Spyder 4.
Just as with the highly regarded i1 Display Pro (D3) you'll not know just how accurate your meter is unless it is verified against a spectro/reference meter.
You'll find that most comments suggesting to avoid the Spyder 4 are from those that have never owned/used one and are basing their opinion on previous generations (mainly the Spyder and Spyder 2).
Best of luck,