I have had the entire Nik Products Suite for several years now and admittedly have a love-hate relationship with the software. Yes, it can do neat things - but on the other hand, it's time consuming (NOT a time saver) and so you have to ask yourself it is really is worth it for the massive time sink in your workflow.
First, 3.0 cleared up nearly all the program-crashing bugs that plagued a lot of Nik's software with Photoshop and version 2.0. The program runs cleanly when invoked and very rarely has it crashed on me. That stability is important: if Nik crashes, so too does your Photoshop. You will lose all work done to that point.
What Nik does is bring some creative effects to the table. The default of the effects (you can download a free demo to try them) always starts to strong. But you can create your own presets within the interface so you can always achieve consistent color effects. That's important since the program goes back to default settings once Photoshop is closed. But with effects from the ever-popular Bleach Bypass, Cross Processing, and color toning, there is a lot to like here. As well, you can control clipping in the lights and darks with many of the effects (or, if you have copious amounts of time, fiddle with the vaunted U-Control points).
So, in all, what it does, it does well. But here's the problem: this program is expensive AND a HUGE time sink. If you don't mind spending 10-20 minutes on each image you create (i.e., you do NOT have a life and don't mind sitting in front of the computer all day/night), this program will give you great images! But most of us, especially those with professional portrait studios, don't have the time. Honestly, almost all of Color Efex effects can be done in LR if you unlock the right combination. And in LR, unlike Color Efex Pro, your images are non destructively edited.
Color Efex has a huge Achilles Heel (in both the PS and LR versions): you can only do ONE effect at a time. Which means you invoke the program, select your effect, play around with the sliders, then exit out. Then you have to invoke it again for each new effect. Just doing a simple toned bleach bypass effect with Color Efex Pro is 10 minutes already - before any retouching is even started. I've come to realize over the last year that as neat as the program is, it's not worth having sessions take 8 hours to process instead of 1 hour with a straight Lightroom edit that doesn't invoke the program.
So here's my assessment: if you are a hobbyist with a lot of time to lavish attention on one single image for 30-60 minutes (or you want to make portfolio images special), then you will like what this program gives you (though I always encourage people to learn LR better anyway). If you are a professional or plan to become a professional who goes through large sessions, then I don't recommend the program. It's just too much of a time sink spent tinkering around with images (and losing consistency throughout as a result). My time is worth too much to spend with a program that won't even allow you to layer effects at the same time.