I've used the M-16 on occasion. OK, on lots of occasions. I've used it in the humidity of the jungle, the arid sand of the desert, and in ice and snow. It was for most of my life my primary weapon.
I won't say that it is the perfect all purpose weapon nor will I say that I was unhappy with it in general. There were times when I wished it had more penetrating power or had a larger ammunition capacity. The failure of magazines has been discussed here and I agree that they left a lot to be desired at times.
Of course, it had trade offs. The lighter weight was a plus while the lack of penetrating capacity of the ammo was not.
Because it's a more precisely made machine, you must perform more maintenance than on some other rifles (ie. AKs) to keep it functioning properly.
If you would compare the 16 to it's biggest competitor, the AK, the M-16's accuracy is a plus. I'm not an armorer or ballistics expert but I can tell you from personal experience that at distances of greater than about 150 meters, involving a soldier with an M-16 and one with an AK-47, the guy with the AK is SOL.
Many of the early problems with the M-16 were the result of bad 5.56 ammo. It was made with dirty burning powder and carbon would quickly accumulate and cause jams. The ammo and maintenance were improved and many problems were prevented. The same can be said for the magazines made for the 16. The spring was poor and ammo would fail to feed into the chamber. This was also remedied.
This is an old discussion but I thought I'd throw my $.02 into it anyway. I'll leave the discussion of other, newer rifles to those with more experience with them than I.