I just got delivery of one today, I was was disappointed with the Spyder 4 and found the manufacturers default monitor settings were more accurate than the Spyder and eventually sold the Spyder.
I use a Dell 2413 and HP ZR24w, both very good monitors.
The Dell has look up tables (LUT) and after finding a very good article on the web about how to calibrate it with the i1 using the Dell software took the plunge and bought one. I've completed the calibration, using preset 1 as a wide gamut and preset 2 as sRGB. The colours and luminance levels seem spot on. I then calibrated the ZR24w (no LUT software settings only) and the luminance levels were spot on but the display had a slightly red tinge to it, so I fine tuned it using manual adjustment, using the Dell monitor as a reference. The 2 monitors are now as identical as I can expect, considering they are different brands and the Dell is wide gamut while the HP isn't.
The i1 did a stella job and the HP monitor only required a small adjustment to have it pretty much looking identical to the Dell.
Bottom line, I can recommend the X-rite i1, especially for those using Dell monitors with LUT facility.
Another bonus, I get £15 off a years sub to Adobe CC, just at the right time too as I'm about to subscribe
Paul