I have a spreadsheet which reaches back into the mists of time, with my weight recorded at least 115 times a year since 2001, and more sporadic entries going back another 10 years and beyond. It also has acquired other data over the the years, like the occasional blood pressure check.
I am consistent when and how I weigh myself, making the weight tracking is fairly routine and accurate. Years ago, I added prediction for any weight loss based on the periods of January, year-to-date, and the last 30 days. This spring, the "last 30 days" prediction was off by a only days as to when would I hit my first goal (17 pounds lost), not bad for the input I gave it.
The blood pressure measurement is sufficient to assure me that I'm normal. I am vaguely amused at how my resting pulse has dropped a little during my current fitness kick, but it's not worth tracking more closely.
The absolute worse metric in the spreadsheet I have is "miles exercised". It started with number of the bicycle miles I had ridden, got the stationary bike added as its own column, then walking, and now elliptical trainer. These are at best apples-to-hotdog comparisons, but I keep it up in part because it serves as a binary flag for having exercised at all on particular day, and that by itself is useful.
Yet, like swooning over cheap sticker awards for losing 5 pounds, I still care about the number of miles. In particular, in the past ten years the most miles I've ever recorded is 602, and yesterday this young year moved beyond the rest of the pack and into second place with 308. By summer's end I could be at a 10 year high. That would be nice.
(Having done ~ 1500+ road miles on my bicycle in 1985, I suspect my all time record is safe for another summer, if not much longer.)