Our 4300K HID kit in the stock Passat lights is still working fine after nearly 100,000 miles. They're aimed well, and we get no flashes from oncoming traffic.
Just yesterday, I drove 400 miles home in our Jetta while Jason followed me in our Passat, and I had absolutely no complaints about the lights, and I'm pretty sensitive to / aware of lighting.
I've certainly seen many HID kits that do not work well in stock halogen housings, and it's certainly a valid concern to raise with this sort of retrofit. However, in this particular Passat, with the kit we purchased, we have had very good results. I cannot tell the OP "yeah, go ahead and do it," because I don't know what he's buying, or what condition his headlights are in. I can only share my own experiences.
Oh, and for the record, HID retrofits are illegal in many localities. If your vehicle is subject to safety inspections, you run the risk of failing since the retrofit is not DOT compliant. This same thing goes for projector retrofits into stock halogen housings.