From a Mac-user's point of view:
Prior to ordering, I spent a few hours reading as many online reviews as I could find, and then watched a couple hours worth of Youtube reviews. The conclusions were nearly unanimous, everyone raves about these speakers. However, I didn't come across anyone using the exact setup I intended for mine--connecting them a 27" iMac. Several reviews mentioned the use of an external DAC (digital to analog converter). In it's simplest form, a DAC serves as an external amplifier between the computer and the speakers. Researching external DACs could easily turn into a career! There are many types, many brands, many functions--and just as many opinions about which one you might need. Fortunately, I eventually came across a thread that suggested iMac's already have a built-in DAC. I didn't (and still don't) know if that's true (maybe all computers have some kind of DAC installed?), but it was enough to decide to order the speakers and try them without an external DAC, first. If it didn't work out, I could always add one later, right?
So, moment of truth: the speakers arrived (with the necessary break-out cable), I unpacked them (they are very attractive, by the way), placed them on my desk (about six feet apart), used the "Y" cable to connect to my iMac's headphone port, and then connected the two speakers. Next, I fired up iTunes and started going through my collection of tunes--and I didn't move for the next THREE HOURS! Holy cow, these speakers are incredible--everything and more than the reviews had lead me to expect. All of my music sounded better than I'd ever heard it sound before (I'm typically a headphone guy). I'm still not sure if my Mac had a built-in DAC or not, but it powers these speakers perfectly--lots of volume, no hum or distortion.
I did come across a nice tip in one of the reviews, a software add-on called BitPerfect. It adds another level of clarity and punch (nothing over the top) to the already great sound I'm getting from this butt-simple, plug and play set up. So, if you're a Mac guy, have no fear, these speakers need nothing more than the included cables, and the computer and music you already have. I still can't get over how good they sound; even music on Youtube will blows me away!
Update: Well it's be a year since I wrote the above and I have made a few adjustment/additions. First off I ended up putting a Jotunheim between the Mac and the speakers (provides a headphone amp and controls speaker volume, but not power). I also upgraded to balanced cables from the Jot to the JBLs. Later I moved the JBLs to a set of floor stands (same general area, just off the desk). End result: overall sound improvement is modest if at all--I can certainly get louder with the Jot, and the speakers are isolated from vibration in theory.
Bottom line is, for a modestly priced system I'm happy with features and utility it provides--definitely wanted a headphone amp for the 6XX and 4XXs I had coming and the Jot drives both quite well. On the other hand, if not for the headphones, I think I could have skipped the Jot and been fine with the Mac and JBLs alone ;- )