I've haven't seen the quality out of a GroBartiG product before, but the 2.5x tele and 0.5x wide seem too good to be true. I once bought a Vitacon teleconverter with a long tele, and ended up being very disappointed with the results. Most 'good' teleconverters have a range from 1.5x to 2.0x, and for wide angle about 0.7x.
What camera are you using?
When I had a Canon prosumer digicam (G5), I searched the web extensively for teleconverters and wide-angle converters. The brands which you should look at are Kenko, Raynox (both lower to mid range), Tiffen (mid to high range), Olympus and Canon (high range). I'd persoally favour the Raynox ones if you're on budget (order from the USA), and the Olympus and Canon ones if you have the money.
I would not even consider brands such as Vitacon, because although they are cheaper and offer more telephoto magnification or wide angle, the quality of the glass is poor. Your resulting images will have visible chromatic aberrations, and the colours will 'bleed'.
Don't compromise on quality. Remember to do test shots before buying, and zoom in all the way for signs of colour bleeding and other irregularities.
A useful links:
You can check out this guy's website. He did a review for G3 teleconverters and wideangle converters. It's
here. His main site has more details and stuff.