I agree with the above that generally there are not huge differences between brands of supplements. I haven't actually had Boost - I get all mine on prescription so it's whatever a particular hospital uses, though my GP gives me a choice when the prescriptions come from her; the availability of different brands also varies from one country to the next.
I've had Ensure and Fortisip, personally I think Ensure tastes best; Fortisips I usually dislike, and can't stand the taste of the Fortijuices (which is a fruit-juice style rather than milkshake style like most other supplements).
Both of these provide all the nutrients you need and are a good source of calories, and I believe Boost is too. There are variations within each brand too - e.g. I know you can get some Ensures that provide fibre while most don't, and of course there are different flavours.
They are good for you, at times I've had up to around 4 a day. I find the limit of how many I can have is determined by how many I can easily stomach. They're not magic - my mum seems to think that because they're liquid they won't fill me up, but I keep telling her this isn't the case! They are filling, so when I have them, I use them as a replacement for food rather than extras. Since they provide all the nutrients you need, they are useful if you have problems with fibre and can't eat much fruit and veg.
I've seen doctors who seem to over-complicate matters when it comes to selecting supplements. I had a doctor who wanted to make me see a dietician in order to work out which type is best for me and to calculate what nutrients I need, etc., etc., etc. But they're all much the same thing as far as I can see - they all provide calories, protein, carbohydrate, fat, vitamins and minerals. I mainly choose based on whether or not I like the taste.
If you only want calories and your diet is providing everything else you need, I think drinking juice, milk or milkshakes, hot chocolate, energy drinks, etc. will work just as well (though there might be other factors related to healthiness, such as sugar content).