Ready to commit to your summer vacation plans? Hilton is launching a Great Getaway sale, which is advertised at up to 30% off hotel rooms for stays between May 28 and September 6.  Good at 2,800 hotels around the world—including Waldorf Astoria Hotels & Resorts, Conrad Hotels & Resorts, Hilton Hotels, Doubletree, Embassy Suites Hotels, Hilton Garden Inn, Hampton Hotels, and Homewood Suites—the offer often includes free breakfast, too, for at least two people.

The discount varies a little by brand name: You’ll get a full 30% off at Doubletree hotels; 25%  at Hilton properties;  and 20% at Hampton Inn, Homewood Suites, Hilton Grand Vacations, Hilton Garden Inn, Waldorf Astoria and Conrad. The deal is available at only select Embassy Suites.

Sounds pretty good, right? No so fast. The discount applies to the Bed & Breakfast rate, which is always higher than the lowest regular rate and the regular advance-purchase rate. And the offer comes with some big restrictions: You have to book at least seven days ahead of your arrival and pay in full upfront, with no possible refund.

We shopped at some family-friendly spots to see how the special offer played out. At the Hilton Garden Inn in Williamsburg, the special offer rate was $112 a night, down from the Bed & Breakfast rate of $141. Now comes the comparison pricing: That's 21% off the B&B rate but just 15% off the best regular rate of $129. Meanwhile, the regular advance-purchase, no-refund rate was $105. Think of the special offer as getting a free, cooked-to-order breakfast for each family member for just $7 in total. Rooms at the hotel, less than two miles from the city center, have fridges and microwaves, and the hotel has the obligatory pool.

At the Hilton Los Cabos resort in Mexico's Cabo San Lucas—which has a beach, pool and kids' club—the special offer shaves 25% off this property's B&B rate. The special offer rate for an ocean view room with two queen beds was $194 a night, compared to $259 for the B&B rate, which includes two buffet breakfasts for the grown-ups each morning and free meals for kids 5 and under. When you figure in those free meals, the special offer probably has the edge over the regular advance-purchase rate of $179 a night—but just barely, and only if your kids are very young.

Verdict: This is a great example of how marketing spin can make a just-okay offer sound much better than it is. And it's a good reminder of why you need to follow recession travel rule no. 1 and always do the math. Sometimes this special offer beats all the hotel's other rates—and sometimes you can do better by choosing another rate from the menu. On the upside, this offer usually delivers a free breakfast, so long as you can also stomach the non-refundable risk.

Look for the Great Getaway rates when entering dates online, or mention the deal when calling 800/446-6677. You must book before July 15. Some brands  have their own twists on the rules, such as a three-night minimum and no breakfast at Hilton Grands, or two-night minimum night stay for Wednesday arrivals at Homewoods Suites. As usual, other restrictions may apply.