I have been big into fitness and lifting for a long time now and have spent a great deal of time dedicated to learning a lot of the science, the ins and outs of calorie tracking, burning, and all that.
From my own observations and from the data that you guys have suggested so far in this thread, in regards to calories burned in particular, the numbers seem to be pretty accurate as to what you could expect from a weightlifting workout.
As a note, I have seen that heart rate can seem rather erratic when wearing the watch for weight lifting, but the truth is that when doing heavy lifting, heart rate can be extremely erratic.
If I were to measure my heart rate manually the second I have just finished my last squat rep after doing 8-10, I would expect to measure my heart rate in the 150-170 area depending. If I were to wait for just a minute and a half, it could have gotten as low as 100BPMs or less depending on how many sets into my workout I am, if I did any warm up cardio or any of that.
From what I understand as the behaviour of the watch, you absolutely MUST have the watch in workout mode for it to display accurate metrics after it's done. The problem if it is not in workout mode is that since it takes your heart rate only ever 10 minutes or so, it could be taking measurement when my heart rate has bottomed out after a set. Or it could take my measurement immediately after each exercise when it is at its peak. This will produce wildly inaccurate results at either end of the spectrum, as opposed to tracking both the highs and lows and giving me what amounts to a mean HR.
Also as someone said, keep the watch on just a little farther away from right where the wrist is to produce more accurate results (there are exceptions to that).
Just some insight for y'all.