Having JUST finished Accumulatorette's A8, including the undercarriage, this is a perfect time for me to chime in
I used to use long-handled brushes like that, including ones that hook up to the hose and have soap-mix capability. But my results were always disappointing. I seemed to miss a lot of dirt and it wasn't nearly as user-friendly an approach as I'd expected. The long brushes don't give me the proper leverage and the large head didn't get into the places I wanted it to. And it was a hassle to rinse/dunk/resoap the thing when lying on a creeper or the floor. I've gradually stopped using them and gone to a different approach, with GREATLY improved results (real
type difference).
These days, I use a brush with a shorter handle (maybe less than 2 feet), usually my Griot's BHB for wheels. I find that if I'm on a creeper, I can reach pretty far with a handle of that length, and that I have good control over it. The smaller head works well and it's generally pretty user-friendly. Most people would probably be better off with one that has (flagged) plastic bristles, like the ones sold for "wheels and grills".
I'll also lift the car up from time to time and get under there to make sure I haven't missed anything. But I usually find it's just fine. The places I HAVE missed needed a different, more precise approach, namely a smaller-headed brush (or even swabs) that can get into the nooks and crannies. I've also been using the EZ Motorcycle Detailing Brush on things like the coil springs of the minivan (run a search for it, TOL sells 'em and we've discussed them to death). It takes a few different tools to really cover all the bases.
In my experience (and I keep the undercarriages of all our vehicles *VERY* clean, do 'em every wash), this is one of those times when the seemingly obvious approach isn't always the best one. But of course, YMMV.
