The wife and I both have Droid X.  I have unlimited on mine and she has the 150 meg plan on hers.  It is easy to monitor so you can track usage to see how close to the limit you are.  If you can spend most of your time on wifi you might be ok but once you get used to using it you might find usage getting high.  With our plan they will allow us to change from unlimited to $15 plan or $15 plan to unlimited if we find out we use it too much or too little.