I just recently switched from SPD-SL to Speedplay Zero. I like a lot of float, and Speedplays are great for that.

Advantages I see over SPD-SL:
1. Ease of proper installation location. Fore/aft adjustment is independent from lateral adjustment (for q-factor), which is independent from foot angle (which is adjusted via the adjustable float mechanism). The ranges of all these adjustments exceed that of SPD-SL style, so it's more likely that you can achieve ideal cleat placement with Speedplays.
2. Speed of entry. A lot of folks refer to the dual-sided entry -- I couldn't care less. What I care about is being able to enter quickly, and I can enter Speedplays quickest.
3. Lowest stack height on the market (distance between sole of shoe and pedal spindle). To me, this results in more "connected" pedaling.
4. Color options so you can bling your awesome style, yo.

Disadvantages:
1. Maintenance intensive. The cleats need to be lubed on a somewhat regular basis. When they're not lubed, it takes noticeably more force to enter. Also, the pedals themselves have to be greased using a grease *** & appropriate grease. With SPD-SLs, I performed zero maintenance on my Dura-Ace pedals in 8 years, and they functioned just as good as on day one. I'll caveat this by saying the Speedplay pedals can be re-built, so overall longevity and cost may be less than owning SPD-SL pedals, but that assumption would take greater than a decade for me to determine.
2. Walking. The cleat base is metal, which makes walking akin to ice skating. I would say purchasing cleat covers are a requirement, not an option. Personally, I use the keep-on covers as I don't walk around in mud, and I don't have to worry about taking them off/putting them on.


For me, the advantages outweigh the disadvantages. When I was doing my research, Speedplay fanboys didn't really bring up the downsides of ownership, so I wanted to point those out so you can make an informed decision.

Edited to add:
My experience with "float" on SPD-SL cleats is that it's not true float . . . it's movement range, but the cleat auto-centers your foot back to 0 degrees of float. So you can angle your foot while pedaling in SPD-SLs, but the cleats will auto-center back to 0.

Speedplays are not like this. Float is legitimate, resistance-free float. The first few times you ride it'll feel very unstable, because your muscles are used to being auto-centered by the SPD-SL cleats, and you won't get that with Speedplays. After a few rides, your muscles will adapt to stabilizing your foot at whatever position is most comfortable for you. This experience of legitimate float can be somewhat disconcerting initially, and it likely what others refer to as "slop." The pedal/cleat interface is NOT sloppy -- it is very solid and secure.