I have a Tanita that gives weight, body fat %, hydration, calories to sustain and bone mass.
The body fat % fluctuates a lot depending on how hydrated you are. When I'm more hydrated it gives a good reading (meaning lower, yea!) but when I'm dehydrated it's higher (poo). Even if the actual number isn't correct I like to watch the trends and find that helpful.
Frankly I'm more interested in the hydration because I can do something about that if a race is coming up. The rest of the info is just interesting to read, from my perspective that is.

I have read that the only way to get an accurate body fat measure is by autopsy. I don't think you want to be doing that on any regular basis, so no matter what you do you're going to get an inaccurate measure.

However, you kind of have to think about accuracy versus consistency. It has been my experience that the scales give a pretty consistent measure as long as you either measure at the same time of day or are willing to do a bit of averaging. Much more consistent, in fact, than pinching calipers or even hydrostatic weighing (both of which I've done, my mom is an exercise science professor, so I had access to all of this stuff growing up). It is affected by hydration levels and things like that, but I can usually spot the outliers pretty easily. I usually see a variation of no more than +/- 1.5% from what I believe to be my actual. I would like, at some point, to validate my scale against calipers and hydrostatic techniques, to get a feel for whether it reads high or low compared to the others.

For me, I got the scale to track progress. Starting last summer I plotted readings in excel and was able to observe a distinct downward trend from 12ish% down to 8ish%. I don't really care whether I actually started at 12 or am currently at 8, but seeing if there was actually a trend was important. I did also observe a slight DOWNWARD trend in the scale's "lean mass" reading, which doesn't make much sense, so I certainly don't believe the readings to be infallible.

My scale (Tanita non-Ironman) does have an "athlete" mode, which it says does the calculation differently. They define "athlete" as working out 2hrs/day, 6 days a week, which IMO is pretty intense for something that's aimed at the general public. I don't think you need to actually be training at that high of a level to use it, but rather just have an "athletic build." I think they were just trying to dissuade everyone who jogs on the treadmill occasionally from entering themselves as "athletes."

I do regret, however, not getting the "Ironman" edition scale. Reason is, my scale calculates DCI (Daily Caloric Intake) rather than BMR (Basal Metabolic Rate). DCI is just BMR multiplied by an activity factor, which you enter when you set up the scale (i.e. 0=sedentary, 4=manual labor day job + daily workouts). I don't ever pay attention to the DCI figure because it tells me I can eat 3600cal/day, when I know that on less active days that's not true. I would rather see my BMR and adjust up accordingly. I suppose I could reset it to "sedentary," but the Ironman scales just show you BMR.

I'd have to agree with GGerhke on the bodayfat %. I have a cheapy hand-held bodyfat device and I'm certain the numbers aren't accurate, but I just look at the trends to ensure I'm on the right track. The caveat being to measure at the same time and under the same conditions...this is usually solved by just measuring when you wake up.

My buddy has the BC558 Tanita Ironman scale, and although body fat fluctuates with hydration, it is a pretty awesome tool. :) The hydration feature is cool. I like it. But then again, I didn't pay the approx $4-500 for it. ;p

The reading on my tanita scale won't display a body fat below 5%. It must be programmed as a built in minimum. I recorded my weight and body fat almost every day last year and I spent 2-3 months with readings of 5.0%. But anyway, the weight is a very nice feature compared to spring operated scales, and as everyone else said the BF% is useful when you are comparing it to past readings. The scale I got was $120, discounted to $40 on Amazon- money well spent IMO.