Depends on what you like to watch. Most of my shows are available through Hulu+ so I am loving that. Also, I'm using Netflix to catch up on old seasons of shows that I wanted to watch but never did. For example, I am going through the first 2 seasons of Dexter now because I just never had time to watch it before and I don't have Showtime. Also plan to go through six seasons of Weeds after that. You should see my Netlix queue.
When it comes time for seasons 3 and 4 of Dexter, if it isn't on Netflix, I'll buy it via Amazon VOD. I've done this for Breaking Bad as well, and will certainly pay the per episode price on Amazon VOD if I don't have cable when the new season begins.
I'm a grad student, so I should be busy working on papers and such, and I have found since owning the Roku, I have wanted to watch more Netflix etc than before.
Before, I usually had the TV on just to be watching something. But now I am watching stuff that I truly want to watch, and thus, will probably cancel cable in the near future and try not to watch my Roku too much.
Other than that, you have things such as Justin.tv if you are the type that likes to stream videos. It appears Justin.tv is starting to offer some higher quality streams, which is good, because a lot of times I stream basketball games that are not available in my area. Having this streaming option for my TV is great, especially if they improve the feeds.
There are other channels that are very specific in their programming, which some people enjoy.
There is also MLB and NHL programming (which costs) if you like those sports.