You're asking two questions here, of sorts, so I'll tackle them separately.
How's the Roku? I had an XDS for about 2 weeks to review back in Sept and enjoyed it, but it had a few shortcomings.
Two things to think about: how much local content do you have (I have a ton) and how strong is your internet connection? To get the most out of the non-local content, I would strongly recommend a wired connection, but if you have to go wirelelss, I would go dual band N for the better picture. Even the 720p content seemed but starved to me.
The hardest part about using it is that you have to have a computer to get the most out of the channels - it's kind of shortcoming in my eyes. You can't pick new channels from the device. But after you've picked your channels, this goes away.
The XDS didn't support all of my codecs for local playback, but it supported most of them. If you were starting fresh with local content, then you could make certain that all local content was created with the proper supported codecs.
The Netflix player is probably the best on the market. I wish Tivo would take note. Hulu+ was not available when I had it, but I hear that's great too.
To your second question - cancelling cable for a Roku. I'm not satisfied with the online selection to completely get rid of cable. YMMV, but for me, sports is the biggest gap. I have gotten rid of Premium Movie channels, but there's still a ton of TV that I watch that is not available streaming. Some folks, in an effort to make up for this, purchased the XDS and bit torrent the gap. That requires the local playback which is why I say of you go the Roku route, I would get the XDS.
