Without details about your application stack, this would be impossible to estimate. Each Heroku application is run in a dyno, which is essentially a Linux container. You really need to read the Heroku reference documentation on their web site: https://devcenter.heroku.com/articles/how-heroku-works
We work for Marketing Customer facing application ... Heroku is chosen as platform to migrate...application is Java & Spring based and I see its one of the supported language/platform in Heroku....As we moving to Agile methodology, we see ready to use platform will ease things...& we can just focus on the application...But what could be some advantages & disadvantages of Heroku in the long run...
Good question that I can't answer since I haven't used it. I understand that Heroku is based upon a Debian/Ubuntu distribution however. There is a lot of useful information on their web site and the documentation seems pretty thorough to me.
Disadvantages: would mostly be whether or not it will survive in the "long" term - long being relative in the Internet era.
Advantages: it was purchased by Salesforce.com in 2010, so that may be a good sign.