There’s no denying that mash-ups are a gimmick. But it’s also worth pointing out that some of the very best developments in the history of pop have been gimmicks—like, seriously, putting a magnetic pickup under the bridge of a guitar? Scratching a record to make a wicky-wicky sound? Fads, I tell you! One of these days kids will return to the good old-fashioned simplicity of banjo music, dammit.

But seriously, folks, I can’t predict how much staying power the mash-up is going to have. All I can say is that in the here-and-now, there are few things quite as enjoyable as a good mash-up, and those who can do it well are geniuses in their own right. Not without reason have I long hailed Richard X’s X-Factor Volume One as one of the buried treasures of the decade so far—Richard X knows what he’s doing, and knows that putting the Sugababe’s “Freak Like Me” over Gary Numan’s “Are Friends Electric?” is simply brilliant.

The faceless DJs who put together The Best Mashups in the World Ever Are From San Francisco definitely know what they’re doing. If sometimes the results are better than others, that only reflects poorly on the occasional conceptual misfire. From a pure craft standpoint, these tracks have each been methodically constructed to precision standards. Putting two disparate and distinctive tracks together to make a seamless whole is not as easy as it looks, and even with the aid of technological marvels such as Reason and Cubase, it still takes an uncanny patience to pull it off.

The weaker tracks essentially suffer from uninspired combinations. A good mash-up puts two or more songs together that can add up to something more than merely the sum of their parts. There’s a dialectic at work here, and if the songs don’t compliment each other in some way, or their interaction is only interesting on a singular level, there’s just no room for the kind of synergy that makes sparks fly. For instance, Axel contributes a track called “Lil’ Brick House” that features the Commodore’s “Brick House” underneath a rap by Lil’ Kim. It’s well done, but not very interesting—yeah, Lil’ Kim is a brick house, OK, we get it. It’s perfectly conceivable that this is a match-up that could happen in the real world. You don’t get the electric shock that you would expect from two totally different substances coming together in uneasy harmony.

But then you have something like Mei-Lwun’s “Sweet Home Country Grammar”, featuring the a capella from Nelly’s debut single over Lynyrd Synyrd’s “Sweet Home Alabama”. This is a track that makes you stand up and pay attention. These are two tracks that most people would never have thought belonged together, but amazingle they do: they are in the same key, in the same tempo, and even share something of a similar relaxed bonhomie. Against all odds it’s a natural fit.

A Plus D contributes” Decepta-Freak-On”, a seamless mesh of the DFA mix of Le Tigre’s “Decepticon” with Missy Elliot’s immortal “Get Ur Freak On”. These tracks are both similar in tone but dissimilar in execution, so the match-up works beautifully. Tripp’s “Maniacs Emerge” mixes Fischerspooner’s deathless “Emerge” with the theme from Flashdance, and the result is interesring but not revelatory. Jay-R contributes a rare misstep for the disc, with “Milkshake It Up”, a mash of Kelis’ ubiquitous “Milkshake” track and the Cars’ “Shake it Up” that just seemd forced, and maybe even a little bit out of tune (rarely a problem considering the technological muscle brought to bear on these hybrids).

Party Ben is the disc’s all-star, with three contributions. The first one fall flat, however, as “Bizarre Light Triangle” is an unconvincing marriage of Madonna and New Order. “Another One Bites Da Funk” combines Queen’s “Another One Bites the Dust” with “Da Funk”, a marriage made all the more seamless for the fact that both songs share an absolutely identical funky disco backbeat. The compilation closes with Party Ben’s “Boulevard of Broken Songs”, which combines Green Day’s “Boulevard of Broken Dreams” with Oasis’ “Wonderwall”, and a little bit of Travis thrown in for good measure. It’s a great track for the degree to which it strays from the simple arithmetic of combining two songs, mixing up different elements to create something a bit more complex than a mere A + B combo.

To that end, however, the best track by far is Earworm’s “No One Takes Your Freedom”, which combines the Beatles’ “For No One” with the Scissor Sisters’ “Take Your Mama Out” and even manages to wedge George Michael’s “Freedom” in there somewhere. It’s a brilliant moment, sure to end up on countless homemade mix CDs and cause never-ending consternation for the lawyers at Apple. This is the way it should be: good music always makes the suits queasy.

There are a few rather tired combos, a few too many tracks with an unrecognizable rap laid over an obvious musical bed. But for every uninspired mash-up that will elicit a chuckle and never be played again, there is another song that will find it’s way into your permanent rotation, an example of bastard pop at its finest and most provocative.