I think the "plastic lets you tell when it's carbonated" is a really stupid "advantage" because first of all, it's not really a mystery...it's gonna take 2-3 weeks at least. What's the rush? Manage your pipeline better.
Second of all, just because the bottle is firm, doesn't mean the CO2 in the headspace has actually absorbed into the beer. The pressure is expected to spike to a high value then actually go down as the CO2 absorbs into the beer. So feeling if the bottles are hard tells you nothing, except that your beer is carbonating; but guess what--if you put sugar in it, it tends to do that.
And third, bottled beer is seldom at it's best the instant it becomes carbonated anyway. I like to let bottle-carbonated beer age for at least a month or two before judging it. 3 months is better. Unless it's a wheat beer or other fast recipe.
I bottled a few batches in plastic soda bottles when I had a big pipeline bubble and severe bottle shortage. Plastic soda bottles work great for beer with the following limitations:
If they held cola or root beer, the flavor will leech into your beer (I confirmed this). Some people say that bleach can get rid of the taste, but to me it's not worth it the effort. I never noticed any taste-leeching from Sprite bottles, but just because I can't taste it, I still think it's there so it's the principle of the thing.
Also, the beer loses carbonation eventually. I found a stray bottle that was probably at least 9 months old and it was pretty much flat.
Other than that, I love them because they are much lighter, easier to cap, and don't break.