A 2.5x powermate was one of the first accessories I bought... I have owned a 2x barlow, I prefer the powermate.
For one, it is parfocal.. ie: you don't need to refocus when you switch from just an eyepiece to eyepiece plus powermate. Secondly, the powermate is optically far superior to any barlow I have ever owned...
well having one of my saturn shots feature on Tele Vue's blog this week for using a powermate (5x) i can absolutely confirm they are definitely good for planetary.
i also have just a bintel 2" 2x barlow which got me some nice snaps (just not that zoomed in as the 5x). but i'd say going over 2x i would want a nice piece of optics like a powermate.
With Jupiter and good seeing and high power, I can firm that the TV 2.5x Powermate was far superior to my old 2x Meade barlow. Although I was pushing at even higher power with the Powermate, the image flatness, control and sharpness was so much better than the barlow. You could tell the barlow was doing 'something' to the image, but the Powermate simply amplified in very clean manner, it's hard to describe. Fox