Problem is battery life and recharge time with both flashes on, you can get the same results using a bounce card and without having to deal with color casts from reflecting walls or ceiling.
The Metz 58-AF-1 has all the functionality of the SB-800 with easier menus and imo superior Metz quality. I don't use strobes that often and I still need to read the manual almost every time when I'm setting up the SB-800, the menus drive me crazy but not with any of my Metz lights.
Battery life? Really? I use flash a lot, and shoot weddings for 6 to 8 hours straight on a set of 2900 mAh batteries no sweat. Yes, bounce cards work (obviously), I didn't specify bounce off ceilings, or walls ... just "bounce." But I do admit that the Metz is more power hungry than the Nikon SB900 even with the secondary flash shut off.
Better quality? "Really?" IMO and in my experience, the Metz quality isn't any better than the Nikon flashes, case in point is that the SB900 has a metal hot shoe and my 54MZ-3 Metz doesn't ... and the battery door to the Nikons is far more sturdy than the flimsy one of the Metz. I wish they'd fix that, it's annoying.
Functionality? forgett the SB800 and compare apples-to-apples. Metz 58 AF-1 does not nearly have all the functionality of the SB900. The Metz doesn't have an off-camera terminal at all. The Zoom head of the Nikon is 12mm to 200mm, the Metz is 18mm to 105mm ... which makes it useless when using a 12 to 24 Nikon lens or a 70-200VR. Not to mention that the Nikon recycles faster than the Metz. The SB900 has automatic gel color ID and auto sets the Nikon camera's WB to match without screwing around with the menu at all. The SB900 has an analog switch for wireless work unlike the SB800 and Metz 58 which makes you go to the menu for that feature. The SB900 has a distance Priority Manual Flash setting which is a God sent when using non-CPU Zeiss manual focus lenses ... etc., etc, etc.
I have a pile of Metz flashes and they are very good, but let's keep it real ... when a maker does a good job with a dedicated flash they should get credit. And Nikon got the SB900 right from everything I've experienced so far.