Being a professional photographer I've been using Nikon Speedlights for about 10 years now. Previously, that's before I switched to all digital, I oftentimes tried a Bronica Medium format film camera and Sunpak, "potato-masher" type flash for editorial and wedding assignments.
Since I switched to all or any digital in 2000, I have used Nikon DSLRs and also the Nikon Speedilights, SB800 and SB600. They are both rugged little flash units with plenty of power.
The SB800 is slightly more powerful compared to SB600 with a guide variety of 125 at ISO 100 versus 118 for the SB600. However the guide numbers do vary with all the degree of zoom used. I get F8 out of my SB800 at 10 feet and ISO 100 with all the zoom set to 50mm at full power. After i add the SB600 at half power, I buy F9.
The SB800 and SB600 are very nearly the same size and weight at 2.8x5.1x3.7 inches and 350grams for your SB800, and 2.7x4.9x3.5 inches and 300 grams for that SB600. And they both have nearly all exactly the same features and controls.
Since I specialize in portrait photography, I personally use the Speedlights when focusing on location. I really appreciate the light weight and small size of the units, as every ounce counts when carrying a full gear bag along a stretch of beach for a half mile, or hiking up a mountain trail to access the locations!
A few things i really like about these flash units is the way these people have a built in diffusion/ zoom system. As all pro photographers know, the smaller the light source, the harsher the light and harder the shadows. However with these little buggers, while using SB800 as my key light, as well as the SB600 for fill, I'm able to replicate exactly the same look as I get in my studio employing a 30x40 inch softbox on my key light along with a 4'x6.5' scrim in front of my fill light!
Generally I'll zoom the SB800 to 50mm, and set the SB600 to 24 or 35mm. The result is a beautiful soft light with gentle transitions from highlights to shadows. Well, actually they do create specular highlights on noses and cheeks, however with just a little air brush in Photoshop, the consequence is beautiful, soft lighting.
I'm an "old school" photographer, and want to control my equipment myself as opposed to let the programs in the equipment control exposure, so I operate everything in manual mode. However, the Nikon Speedlights possess a built in remote trigger system that also sees the light output and makes adjustments. This feature is called the Creative Lighting System, or CLS.
Reading the manuals and trying to use the CLS was not as successful for me personally as I could have liked. I have been unable to learn how to use the CLS with all the flash units in manual so I can control the output, although I pair the lights with Nikon cameras. However, you will find others who figure out they have been able to do just that.
My biggest complaint in regards to the CLS is that it's an inferred based system, and then the 'slave' flash(es) must begin to see the inferred beam coming from the 'Master' unit, and also the sensor is found on one side from the flash. Consequently, the units has to be in line of sight of each other. And, because the master flash sends out a signal pre-flash, then computes the data before firing the 'actual' flash, it delays the exposure. As well as in portrait photography a lag between pressing the shutter release and making the exposure isn't a good thing!
Both the SB800 and SB600 have swivel and tilt heads that allow you to bounce the light off of surfaces, and easily direct the light where you need it. In manual mode you are able to fire them at full, (1/1), power, 1/2, 1/4, 1/8, and 1/16 power. There is also TTL modes you can use with Nikon cameras who have CLS capability, and for those that don't.
These flash units are hotshoe mount flashes, and the SB800 also has a PC plug input. They both come with little stands, or rather "feet" that are a set platform with 3 hotshoe-like docking slots for mounting the flash. In this way you can attach them to a light stand as on the bottom of the foot there exists a standard threaded nut for this purpose. You can also place the flash over a counter, shelf or perhaps the floor by technique foot platform.
They are powered by four AA batteries, and recycle in about 5.5 seconds at full power. I get about 120 full power flashes using one set of batteries. I take advantage of the rechargeable batteries, so I get about 120 full power flashes before having to recharge the batteries. It comes with an indicator light about the back from the flash to show you when it's fully recycled, and the recycle time does slow considerably because the batteries get low.
Both flash units possess a digital LCD readout with the settings about the back, and also the controls are very intuitive...specifically for use in manual mode.
I have found the light output to become very uniform and consistent, which is a definite asset to portrait lighting. And as I said earlier, I enjoy the built in diffusion and zoom capabilities. Add to that the light-weight and compact size, and durability of these units, plus you've got an outstanding location portrait lighting kit!