Definitely worth it. I have the MP-20 on my 20g, and I've bought the MP-40w upgrade from Aquatic Addictions for when I upgrade tanks.
The versatility of the small pump is second to none, and the amount of control you have over it is amazing - I run mine at 100% reef crest for an SPS dominated tank.
The second con (after initial cost) is that the flow can't be directed like conventional powerheads - I'd love it if somehow I could mount it in the corner to flow diagonally, but meh - the flow itself is so broad, I'm not sure how much of a difference it would make anyways.
If you're looking to pick up two and a battery back-up, Marine Depot (and I'm sure other online places) has a package of 2 MP-40w and a batery back up for just over $1000USD - after conversion it works out to just under $400 CAD for each MP-40w (not including taexs and shipping though) - Maybe a local retailer or sponsor can match that for you
