Question #1- Do you care if the shows you are watching are first-run? Most shows on Netflix are the ones that have been released to DVD. But yes, there are many good TLC/Discovery type shows. There are also many good dramas/comedies, etc for Netflix, again, if you don't mind watching last season's shows. Occasionally they'll have a series that will play a week behind the broadcasted show, but not typically. You can get same-season shows on Amazon VOD, but you pay to PURCHASE each episode, at around $.99-$2.99 per episode. The nice thing is you get to keep the "digital copy", and watch it whenever you feel like it. Often, Amazon has deals on entire seasons (not current seasons though), for under $20.
Question #2- Do you have a nice fast internet connection (1.5Mbps or faster)? If so, you're in good shape.. I think 3Mbps is a more realistic minimum for HD. Plus, Roku just made their high-end box 1080p compatible, so the faster the connection, the better.
You get a lot of variety with the Roku.. I still haven't "cut the cord" because there are too many broadcast channels that I record using my HD cable box, and my girlfriend watches a LOT of E! (unfortunately!).. I suppose if Amazon offered a "pay for this show in advance", and I could automatically buy or maybe rent shows at the beginning of the season, I'd be more likely to get rid of cable. For example, I watch The Office every season, and will until it goes off the air. If Amazon gave me the option to "pre-pay" for the season at a discount, say for $25, I'd do it.. I could get my 5 favorite shows for $125-$200, for the entire season.. Which is what I pay PER MONTH for my cable right now. Ouch.
Yeah, I got some Rokus.