Great to keep in the bag
Will this get the same results as a dedicated 400/500/600mm prime lens..?
Nope.

Did I expect it to..?
Nope.

It's a great thing to leave in the bag for those times when 200mm is just not quite enough or you didn't feel like lugging the 300 f/2.8 (or longer) around.

OK, so it drops my 70-200 down to f/5.6 but I can live with that if it means the difference between getting a shot or going home empty handed.

The difference in sharpness in prints is only slight and only found IF you're looking for it. Most people will never see the difference. That said, care needs to be taken. Using a tripod is almost a must. (FWIW I use one a lot and for ANYTHING shot at 300mm or greater.) Hand holding a 400mm lens is a bit dodgy at the best of times IMO.

Most of the complaints I see for this are by people that either didn't take the time to check which lenses it will work on or figure that they'll get exactly the same results using a 70-200 f/4 (or lesser) as they would if they had chosen a fast 400mm in the first place. Not going to happen. Canon is very specific about which lenses can use this...and the body focus specs aren't exactly hidden either...

If you're looking for a quick and easy way to increase the reach of an already SHARP compatible lens then go for it. If you think you're going to get Canon 400mm f/2.8L performance for [...] bucks... Well, sorry but, time for a reality check.

So why 4 stars instead of 5?
I don't want to skew the results and make people think this is something that they must have or can't live without. IF you can use it, know it's limitations, and HOW to use it...well then it's a nice addition to the kit. OTOH, it's not the end all be all accessory that many people hope/wish/think that it is.

Like I said up top, it's a great thing to keep in the bag.

This may over ride auto focus capabilities
I shoot a lot of long distance action, sports, wildlife, etc. and thought this would be a good addition for my canon 100-400mm. My long canon lens is an f/5.6 and when multiplied by the 2x extender gives it an equivalent of f/11.2, beyond the f/8 capability of the autofocus. It worked fine on manual focus and for many situations, this is actually the best way to go, but for on-the-move photography (birds in flight, football players)it was inconvenient. I ended up sending it back and am going with the 1.4 extender - it does not provide as much range, but does not override the auto focus either. The lens is excellent, just beware of the limitations it may impose on your particular needs.

canon EF 2x teleconverter
Great converter, but very limited to number of lens that it can be used with.

VERY UNSATISFIED
At first I was exited to get this product, particularly because it was a Canon product that would compliment my Canon EOS Rebel camera and EF lens. However, I was very disappointed to learn after the fact that the EF 2X II extender is NOT compatible to all Canon EF lenses, in my case...the 75-300mm EF telephoto lens. This was a significant detail NOT indicated in the Amazon advertisement. When I contacted the vendor (DB Roth), I explained that I was not aware the product did not fit my lens. I felt like I was brushed off and simply told to call the manufacturer. Long story short...I called Canon who tried to help, but they confirmed there are in fact various Canon lenses that are incompatible. In the end...I went back to the vendor (DB Roth), who reluctantly agreed to give me a refund. However, they accepted no responsibility for NOT stating incompatibility issues in the Amazon advertisement and had the audacity to charge me a restocking fee, which they deducted from my refund!

Canon 2x Extender
I love this addition to my camera bag! Works get with my 70 - 200 zoom. I only wish it would work with more lenses.