I gotta agree with Bob mostly,
The TV stuff is designed to work especially well with fast focal ratio scopes, not all eyepieces &/or barlows can state that, so they do have their importaint nitch with guys who have HUGE reflectors running at f/4 etc. I do think however IF you own a real APO refractor that one should use a triplet barlow or a Powermate to keep color correction perfect if your into looking at star color differences in doubles & for hi end Planetary colors "especially if doing AP" etc... In regular reflectors & achros eh,............a regular 2 lens barlow is more then sufficient for visual.
I do get no blackouts or kidneybeaning with my Powermate which I have seen on cheap barlows used with cheap plossels so its a keeper.
To answer your question tho, Powermates are basicly mostly used for AP but are also fantastic used as a barlow. I had a triplet barlow & tested against my Powermate the Powermate was better in some areas, mainy the eyerelief stayed better & no blackouts at hi mag etc...The Powermate retains the full FOV & eye relief of whatever eyepiece you use, not all regular barlows can state that.......................... so you get what you pay for its true.
I bought mine at 1/2 price so it was a no brainer decision that I don't regret.
If you go to the TV website they can explain the difference between a barlow & a powermate they aren't the same animal. The powermate restores what a barlow can take away from an eyepiece.
Dave
Z 8"/ 8" Baader SF/ 12v powertank/Dewbuster
4" DIY SF/ 1.25" OIII filter
TV32plossel/GSO10,15,20,25,SWF
Starbound chair, Vixen 6mm & 15mm NLVs
Pentax 7X50, TV 2.5 X/Z 2" 30mm WF/C 6.3FR
C6SE / Celestron Onyx 80EDF / Cube Pro GPS / 12.5 & 9mm UO Orthos