Yes it would. The Now Stick was actually made for Now by Roku, so it was practically identical to the Roku Stick of the time. The changes made for Now were to add the Passes (now Memberships) handling, restrict the apps that could be added, and include Sky Box Office, for pay-per-view boxing and the like.
And also to make it start up in Now; though if you arrow out of Now, you will see the Roku-type UI where you get the other apps.
So a Roku Stick (or a Roku Express) will give you everything a Now Stick currently offers, plus an enormous number of other apps (called channels) you can pick from, though not Sky Box Office, but Now will be just one channel among many to choose from.
Though you can arrange the apps you have in any order you like - they appear in a rectangle, 3 wide and as deep as needed - so you would probably want to put Now top left, for easy access.
Amazon Firesticks can also bring you Now, with all the features this app has today on a Now Stick, and lots of other apps also, but it doesn’t have the Roku focus on doing just that and only that, offering lots of movies and shows intermingled.
Imagine the Now screen you have now, but it has a row or rows of apps intermingled as well, and you will get the idea. My personal preference is the way Roku do it, but up your mileage may vary 😛
Yes your existing NOW account will work on the Roku NOW App, just sign into the NOW App on the Roku by clicking on the gear wheel icon on the NOW App on the Roku with your existing NOW login details.
The NOW App on the Roku looks the same has using a NOW branded device when either watching on Demand movies or TV shows or watching live NOW content.