I read that the current should be at least 700 mA. I plugged a power supply with output current of 550 mA, and it seems to work fine. Can something happen to my Raspberry Pi? Does a wrong current influence the performance?
The 700 mA recommendation errs on the side of caution.
The Raspberry Pi itself needs around 400 mA. Powering a typical basic keyboard and mouse needs another 100 mA or so. You should be fine. But if you plug anything in that needs some serious power like a Wi-Fi adapter, keep in mind that you may be pushing the limits of your power supply.
One of the most annoying problems you can have with an underpowered supply is that it works fine until something draws a little bit too much current, then the supply dips, just briefly, and corrupts the RAM. A few seconds or minutes later, that corrupted RAM location is read (wrong) and some part of a program goes wrong.
It can be a nightmare to track down!
Most likely some USB peripherals will not work because they will not have adequate power. Do you have a keyboard and mouse connected? If so you may have been lucky to find ones with low power requirements (please list them on this wiki page!)
I am also guessing you do not have a USB hub or flash drive connected. There is a good chance that they will fail to work (and possibly crash the Pi.)
I can also report my Pi works fine in the above scenario (this is with just a normal keyboard / mouse attached, nothing fancy). If you start adding more current drawing peripherals to the USB ports you may start to notice issues.
You do technically run several risks this way, which may range from everything from minor stability issues to permanently damaging the Pi or the powering device (I'd say unlikely, but possible - it all depends on the tolerances involved.)
The 400 mA minimal cited by Nick McCloud seems to be right. I just plugged my Raspberry Pi on a 300 mA power supply and it booted, but I noticed that it couldn't power the Ethernet interface (the Ethernet LEDs started to power on and off every second or so).
One problem you may run into with insufficient power is memory card corruption. SD cards can draw significant power on writes (relative to reads), and the amount of power used while writing varies a lot between different models of card. Thus, someone else using the same configuration except for the card may get along just fine with a power supply that wouldn't provide enough power for you.
You can get small meters that fit in-line in your USB power connection that will show how much current is being used, and this can be helpful. However, these usually don't show short term peaks in current use and thus may not detect the peaks that could, e.g., lead to memory card corruption.