You mean ones with a transformer, as well as the different physical connection?
Well, if it only converts voltage, you need to beware of the frequency difference between US and Europe (60 and 50 Hz, respectively). Items with motors or oscillating parts will act differently, and may overheat. Clocks that time off of the wall current will act differently too. Otherwise, most items will work pretty well, so long as you do not draw too much power. Just look to the label on the converter, and see what the Watt rating is.
It is getting easier and easier, BTW, to find new appliances that work on both voltages/cycles. We have a hair dryer and a curling iron, for example, that work on both. And my laptop doesn’t care about anything, so long as you can fit the plug physically into the socket (because it converts it all to DC anyhow).
I haven’t experienced or heard of major problems. You do need to check if the converter has enough capacity (wattage) for the appliance - small ones are only good for about 100W. And watch out for the 50Hz/60Hz difference as Anthracite said, though in my experience, it’s only a problem with electro-mechanical (?) timers like those on toasters, and big motors on washing machines and fridges. No problem with stereos, TVs and computers.
Not nowadays anyway. Back when I was a kid, all the C=64’s had their real time clocks based on the line frequency. I was also into, uh, not getting my software through strictly legal channels (at the time). It was always fun getting European versions of games that ran 20% faster on our machines than they were supposed to. I think that was the first form of overclocking!
For the technically minded: no, the computers didn’t really run at different speeds, but action games (the kind that use the clock for timing) TIMED at different speeds, giving the appearance of fasterness.
I assume we are talking here about adapters, not transformers.
There isn’t much danger but I would suggest getting good quality adapters that fit well (on both sides). If you have a poor fit, or poor connection, you can get overheating which can lead to a fire (but this is also a risk with any bad plug).
The biggest danger is with adapters that let you plug 110 VAC plugs into 220 VAC outlets. I have, more than once, accidentally plugged a 110 VAC appliance into 220 VAC. Most 110 VAC appliances tolerate 220 VAC very poorly.
I spent 2 years in Germany, but we had regular, good-ole 110VAC/60Hz wired througout the dormitories, so I never really had to worry about it. But, let me present this for questioning:
How do the Europeans (and other 240VAC countries) actually wire their outlets? Here in the USA, all (or almost all) houses are wire for 220VAC via center tap neutrals. This means your grounded-neutral gives you ?110VAC x 2, or ?220VAC when you use both the high and low taps on the transformer (thus we get 220VAC for washers and stuff).
How does the earth plug work in other countries? Is it common to the 240VAC “neutral” or is there no “neutral.” Can you get just ?110VAC out of European outlets by (unsafely, mind you) using just the ground and a SINGLE “hot” lead?