Prices of portable GPS systems keep coming down. But not enough to make for $99 portable GPSes--not quite yet. If you want cheap, the lower limit will likely be $120 to $150 as of early fall. Come 2009, you'll see $99 units. And maybe on Black Friday, the day after Thanksgiving (when stores make so much money their red-ink losses become black-ink profits for the year), you'll find a $99 unit. But are you sure you'd want a $99 unit? Here are five reasons the cheapest devices aren't worth your time:

1. Small screens. Most likely they'll be 3.5 inches, not 4 inches and up. The smaller the screen, the harder it is to see the maps, and the smaller the text. This is an issue if you don't have perfect eyesight.

2. No spoken street names. Every GPS navigation system talks to you. Only some convert all text to speech and speak out all street names, which is a big plus. "Take the second turn on the right" is not the same "as Take the second right onto U.S. Highway 101." A few speak out the names of interstates, but not Sycamore Avenue or Blessington Street.

3. Lousy displays. No GPS device works as well as it should on the dashboard on a sunny day, and that may not change until higher-end units adopt transflective LCD screens that use sunlight for illumination, not just a backlight. But some transmissive LCDs (what we have now) aren't as bright or glare-resistant as those on costlier GPS systems.

4. Less memory, fewer POI. With less onboard memory, there are fewer points of interest stored, and less chance you'll find the hotel, restaurant, or ATM you're looking for.

5. Old models, worse features. When new GPSes arrive, the old inventory may be cleared out, but they often lack the features mentioned above. The new model may have fonts big enough to be legible; the old one may be too small to read while moving. And the old models may be annoyingly slow to calculate and recalculate your route.

Of the portable GPS reviews written this year by PCMag.com, the units receiving a rating of four and above (out of five) cost $300 and up. The one exception is Verizon's VZ Navigator 4 , which isn't a GPS but a $10-a-month add-on to most Verizon phones. Bottom line: Every GPS navigation device gets you where you're going eventually. Any GPS is better than no GPS. But the cheaper ones make it more of a hassle, so much that you'll perhaps wish you'd shopped for a $250 unit; not one that cost $150--or $99.