There's three main contributions to GPS accuracy:

1) WAAS capability - without it, no better than 10 meter accuracy. With it, 3-5 meter accuracy on average.
2) Antenna - A patch antenna is pretty omnidirectional, but only something like 50% as efficient as a Quadrifilar Helix when the Quad Helix is oriented correctly. This of course means sometimes the patch antenna actually tunes a better signal, but the potential is higher with the Quad Helix.
3) Chip design - There's a variety of GPS chips out there and they are not all equal. The latest, greatest are those with the SiRF chipset. Those have the quickest, most accurate signal acquisition and tracking, especially under cover and in obstructed areas.

The reason I bother pointing all this out is that generally the Palm-Pilot and cel phone style GPS will be acceptable for automobile driving directions, but it will never be as accurate as dedicated units like the high end stuff from Garmin. It's up to the user to decide what fits their budget and demands of course, but remember that not all GPS receivers are equal.