The master collection will run you $2100 (
source) so at $50/mo, it will take you 42 months to spend the same amount. As of CS5.5, Adobe moved to a yearly release cycle. Given that you get updated apps automatically (as far as I can tell), this actually sounds like a deal.
I think
this Ars article sums it up nicely: "An individual license for Photoshop CS6 will run you $699 at full price, but a two-year subscription to the software at $20 a month will cost only $480. The Master Collection of the suite goes for $2599, but a two-year subscription would run you a comparatively meager $1200." And really, in the grand scheme of things, how will this make any difference in the day to day experiences with the software? You save money and still get to use the software without limitations.