Typical web, IM, gaming, etc. is easily handled with ~1Mbps per device.
150Mbps is crazy overkill for a single family. 150Mbps (if it were symmetric) would be something a medium size office environment (100+ employees) would make good use of.
I would suggest a ~25Mbps plan.
(This is mostly for giggles, but I strongly disagree with the 1Mbps figure.)
As another poster mentioned, the number of devices is a poor measure of what you really want to know, which is how much simultaneous usage there will be.
Netflix at 1920x1080 is dynamic now (or at least they're transitioning their catalog now), but averages 5Mbps.
Music, at fairly high quality, is ~300kbps, so it's irrelevant.
Where I disagree is using 1Mbps for browsing. If you do browsing with few or no images, this will be fine. But it's not uncommon to run into 2MB images, which take 16 seconds each (ideal) with 1Mbps. Browsing, however, is an activity that can handle many parallel users because you want burst speed (as opposed to the sustained bandwidth needs of video). I'd say that 20-25Mbps is the level at which facebook browsing (an image-heavy example) is unlikely to improve much more, and probably 5 users could happily share that.
Beyond 25Mbps is for file downloading (or for many users). Torrenting, backups, game and movie downloading, etc. In some cases, you get stuck with more download bandwidth than needed in order to get upload bandwidth (because some carriers give wildly asymmetric up/down, like 60/5). This mostly matters for file sync (Google Drive/Dropbox.)
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Damn ye whippersnappers are your fancy panty intertubes speed. When I was a youngin' we had 9.6Kbps and were damn spoiled with that!
Also, get off my lawn!
;)