This is a very loaded question in my opinion – so I’ll break it down into a few different parts.
As far as effective at cutting a dog’s hair to the desired length, with a clean even look, then I would say the Pet Flowbee does a good job. It is easy enough (and safe enough) that just about anyone can use it. It really is a wonderful device!
On the other hand – if your pet is afraid of the vacuum (or any other noisy appliances) then this is likely to scare your pet. They are likely to run and hide the minute they see you pull this out. All that can make for a traumatic event for your both you and your dog alike.
Another thing I want to mention is that some dog’s hair might just be too much for this device. For example a dog with lots of thick hair (like a “Lassie” dog) it may not be as effective as on say, a Yorkie with finer, thinner hair.
You may be able to get around that last issue by limiting the amount of fur you try to trim at once. There are a lot of mixed results and some of them do claim that their dog’s hair looked a little “choppy” after the trim. I think this could be avoided with a proper technique, however.
For example, it says to use an “up and down” motion so the hair gets sucked straight up into the vacuum. After you’ve gone over your whole pet and you have everything at a decent length, I would go over once again.
The second time around my suggestion would be to use it like a “comb” but go against the direction your dog’s fur grows. (Up from tail end to head.) This may help even out some of the “choppy” look that those reviews mentioned.