Bargain hunters beware: Looking for a low price online can be easy, but you need to comparison-shop the tools before you comparison-shop the stores.

The much-hyped tools to check prices at many different Web sites--now often included in the search services of major Web portals--are improving, but they still rarely work as advertised. It's increasingly hard to tell how many retailers a tool searches, the freshness of the price information and whether merchants were excluded because they didn't pay for favored status.

The news this holiday season is the advent of a second generation of "shopping bots"--clever software robots that scour the Web to scoop up prices, shipping fees and delivery times. These newer "bots" travel with shoppers as they surf the Web, running price checks in the background and displaying competing prices in horizontal tool bars that float on the shopper's desktop.

Two of these young hot bots, R U Sure (www.rusure.com) and Dash (www.dash.com), rely on software that shoppers must download and install on their computers. Dash's specialty is discounts; it offers rebates from participating merchants and tells shoppers when it finds merchants (who usually pay Dash a fee) that offer better prices for products the shopper is examining. R U Sure pops up automatically when a shopper puts an item in a shopping cart and then--with sound effects such as the "ding!" of a cash register--displays other store names and prices in a scrolling ticker.

Neat idea, when it works. In our tests, R U Sure's tool bar popped up at unpredictable moments and refused to pop up when summoned at other times. And this bot only searches about 200 e-tailers, a fraction of, for instance, the 2,000 merchants searched by one of its competitors, mySimon (www.mysimon.com).

One thing to keep in mind is the referral fees these search sites ask for; mySimon chief executive Josh Goldman said the company asks for less money from merchants in order to cover more sites.

MySimon is the best of the bots at the moment; it searches not just price but also such features as shipping fees, and it can save search results and send e-mail alerts when prices drop.

Other bots search more narrowly. Cnet's shopper.com (shopper.cnet.com), for instance, checks prices for electronics. For books, try E-Compare (www.ecompare.com), BookFinder.com (www.bookfinder.com) and AddAll (www.addall.com). DealPilot (www.dealpilot. com) will check prices on books, music and videos. (Disclaimer: Washingtonpost.com runs its own comparison-shopping service, based on technology developed by Inktomi.)

Several bots compare online auction prices, but the premiere Web auctioneer, eBay, recently warned its operators to stop retrieving its product listings and blocked AuctionWatch's agent (www.auctionwatch.com) from "crawling" the online flea market.

But before adding yet another bot to their bookmarks, perplexed consumers would do better to visit a different sort of comparison-shopping site--BizRate (www.bizrate.com), which tracks not prices but customer satisfaction. The site collects feedback from shoppers (which it also sells as market research data to Web companies), lists online stores' features and policies, and channels rebates from merchants to consumers.