It's very reliable and is the defacto method of payment in places like pubs. Often I'll hand my card over and they'll ask 'Can I just PayWave that?'
Here (Austria) that's mostly because some people have cards without PayPass and they are not aware that they need new cards for it. I never had problems with it and always use it (even for transactions > 25 EUR) in which case it prompts for the PIN. It's super convenient and much faster.
Probably not a big problem but I have a tendency to sit on my card every so often.