There's something, I don't know what, that makes this better than the other 66's available. I like it. I think you ought to buy this.
The livery is accurate, the cab feels right and the control systems are (quite) accurate.
Good model detail
Control systems - centre sprung brakes etc
Train Length button
Nice (ish) scenarios
The horn... ouch.
Could use a better (slightly less obvious) particle system - I've not seen a 66 putting out that much smoke, especially at idle. This is a complaint I have with all the 66s available.
Headlight control - only off/front/rear
TL;DR: This is a nice rendition of the class 66. If you're planning to use workshop scenarios with the 66, I've found there to be more available for the Freightliner livery version. However I actually prefer this version, and I think you should get it. But you should also get the Armstrong Powerhouse sound pack, because reasons.