I like to run 2 prisms and 9-10 Reapers as my Heavy Support choices, and I've done it several times in 8th ed now, so lemme give you my opinion:
Two prisms generally work great so long as you always use the stratagem, and never move them more than you're allowed in order to fire twice. A single prism, unfortunately, is just a waste of points. That's the short version.
To go into detail:
The damage output of two prisms (using the stratagem and firing twice, of course) is great, no matter which weapon profile you choose.
The strength of the prism is its versatility. Note that if you want to use them for a specific purpose, there are better choices. That's the main thing to keep in mind.
For instance, I use mine mainly as tank hunters, so in theory it would be better to just take war walkers (4 WWs with lances cost just a little more than 2 prisms). However, every once in a while I get lucky enough to take out my opponent's tanks early on, and then the versatility of the prisms come into play as I can just start using another weapon profile and keep on blasting things off the table.
It is important to note, though, that as soon as one prism is destroyed, the power of the remaining prism drops exponentially (due to no longer being able to use the stratagem).
In other words, how much value you'll get out of your prisms depend on your ability (and your opponent's inability) to take one out. As an example, one of my opponents recently learned just how worthless a single prism is on its own that after he takes one of the two out, he just ignores the remaining one as it's only a minor threat from that point on.
I'll never take a single prism in this edition, but I'll still gladly take two.