EVFs aren't as good as a reflex viewfinder - yet. The G1 has got around the lack of a mirror by using one but, from the reports that I've read, it isn't quite 'there' just yet.
Potentially, EVFs could be better than a mirror. My Canon S3's EVF, with about 115,000 pixels, isn't good enough. My Panasonic FZ30, with about twice that, is much better. I haven't used a G1, but with twice that again I image it is, as you say, nearly there.
The G1's 480,000 pixels is just 800x600, so surely we can expect that count to be doubled and doubled again eventually, then surely it will be "there".
In the meantime, EVFs have some advantages over mirrors already. Many cameras have a manual focusing mode that magnifies the centre portion of the EVF as soon as you start adjusting the focus. You can add a "critical focuser" to many SLRs, but not instantaneously like that.
An EVF can also boost the brightness, making manual focusing possible when it might be too dark to do it with a mirror system.
I guess though, that a mirror will always have the advantage that it uses no power.
I think some EVF cameras have put people off them by preventing you from composing the next shot while the previous one is being processed. My S3 isn't too bad for birds in flight, because it has a live display during processing. But the FZ30 is useless, as it freezes till processing is done. But they are both old cameras, perhaps all EVFs show a live view now.
