Who's my mommy?  The Leica SF-24D has very obvious Metz heritage (well, how many flash manufacturers exist in Germany...).  It has the virtue of perfect flash control and consistent power output.  It is well-built and has a pleasing design.  That said, it also has the modern Metz vices of fairly poorly-thought-out controls that have the tendency not to do anything until you hit a button twice, primitive LCDs, and strange markings on controls.

Form: the SF-24D is a relatively small flash, about half the size of a Nikon SB-800.  Its compact size comes in part from the fact that the head does not tilt or swivel.  It weighs a few ounces and slips into a pocket fairly easily.  The head does not project much from the body, just enough to mount a wideangle diffuser.  The SF-24D does a decent job of separating the flash from the lens axis, as much as any other compact I've seen.  The flash connects to the camera with a pin-locking hot shoe connection (which, by the way, really would have been helpful on the Universal Wideangle Finder M).

Exposure modes: The flash has three basic modes: TTL/GNC (a real misnomer, since Guide Number Control has nothing to do with how this mode works), A (automatic) and M (manual, where you might actually use guide numbers).  There is nothing unusual about these.  The TTL mode is generally the one you want, automatic is generally for when you use the flash with a non-Leica camera (or one that lacks TTL), and manual is for very special occasions.  Like when you finally figure out how to bolt that GN Nikkor onto your Leica.  Or when you repurpose your Leica flash for your Hexar AF.

Bounce this: there is no bounce capability on this flash.  Nor is there any swivel.  This puts the SF-24D in the same functionality capability as a $12 Vivitar 2600.  It does, however, have as much power (GN 24 @ ISO 100 /m) as a $17 Vivitar 2800.  Ok, that's not a fair comparison - but I have yet to identify what Leica or Metz solution would ever give TTL flash in a bounce setting.  Good luck figuring out what Metz's bounce-capable Mecablitz units and modules even work with the M8's TTL mode.  If any actually do.  The good news is that a Nikon off-camera TTL cord (like an SC-17) works fine with the M8 and SF-24D.  No electronics inside - just straight wires - and all the pins are in the right place to interface at each end.

Pop-Pop: On the M8, the TTL exposure control is quite good.  ISO carries over to the flash via the hot shoe of the camera.  That said, there is an excruciating lag between the preflash and the main flash.  I have only encountered one human subject who had fast enough reflexes to blink between the flashes  (that's you, Mr. Garcia), and he managed to do it six times in a row.    If you know Louie, are in a hurry or have critical action shots, consider shooting in A - or skipping the SF-24D altogether in favor of a Nikon SB-20 that you can buy on Ebay for about $50.  For all the irritation, though, the SF-24D's TTL mode is very hard to beat.

The battery system: Metz, like many German manufacturers, enjoys making "improvements" in ways that do not address any real problem, present some theoretical advantage, are of little actual benefit to the end user, and result in increased costs of operation.  If you know what iDrive is, I think you understand this.  Metz seems to have a long-term fixation with making improvements in batteries, summarized for your convenience as follows:

"We hate AA batteries.  So we're going to give you NiMH/dryfit/lead-acid standard." (c.f. 60CT-4, 45CL-4 Digital)

"If you use AA batteries, you must use an unusual number of them (like 6) that is not easily divisible from a normal package of 4, 8 or 16." (c.f. 45 series battery holders)

"If it's a compact flash, you must know that it is technically impossible to use AAs in it.  If any other company does, it must be staffed by fools.  You should use a pair of $13 CR2s instead."  (c.f. 34AF-3)

Well, the SF-24D (like the SF-20) is no exception.  Two CR123As.  At best these are $10 a pair. Granted, they last for a while, but come on.  The saving grace is that unique among the expensive lithium batteries, the CR123A has a lithium-ion equivalent that sells for about $25 a pair with charger.  The rechargeables are a great solution, cost about $25 a set, and work fantastically (just make sure that yours do not deviate too much from 3V when charged).  Paris is well worth a mass, I guess.