Some people Worry (for shame!-) that a bottle filler causes more oxidation (because of spraying through the smaller opening, rather than through the whole opening at the end of the siphon tube, I guess). The solution is to tilt the bottle at the beginning, and stick the end of the bottling wand into the bottom "corner" of the bottle. The wand's end is soon covered with beer, and no amount of spraying will cause any extra air to be mixed in with the beer. Also, if the end of the wand (or siphon tube, or whatever) isn't much lower than the end of the siphon tube in the priming carboy (or whatever), the beer will be siphoned slowly, at low pressure, reducing spraying. (This works for any bottle filling procedure.)

Another problem is the bottle filler has beer in it. When you lift the filler from the bottle, that beer doesn't go into the bottle, and the headspace is greatly increased. Even if you fill the bottle almost full, the resulting headspace is larger than some people consider optimal. You can fill the bottle, move the filler to the top of the bottle, and press the tip of the filler to drizzle enough beer down the side of the bottle to reduce the head space.

There are two kinds of fillers. One kind has a spring. The other has a stopper that's held down by the weight of the beer. The latter is slower. Does that mean oxidation is less of a problem? I expect it's easier to finish filling (using the side-of-the-bottle trick) with a springless filler.

(There's also something called Phil's Philler, which has a hole at the top as well as at the bottom. You can remove the filler without removing the beer in it, thus eliminating the headspace problem.)

My take on all this is that there are ways to use a bottle filler to reduce problems (and reduce Worry). I didn't find enough evidence of problems to bottle my beer without a bottle filler.

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