If you miss a payment, or stop making payments, you no longer have a valid use license/license key.
And at $20 a month you're signing up for a 1 year plan.
You are only paying for access, and will spend $240 a year.
Those that had/have Photoshop CS 5 only have to pay upgrade pricing to get CS 6 - $199 - and a valid use license/license key that has no expiration date.
If a new release comes out every 18 months, in 3 years those that upgrade will spend $398. Those that do the $20 a month cloud plan will spend $720. You can hear Adobe's cash register go - Cha-Ching! - every time you make a payment.
I still have CS 4 Extended, and fire it up when I have need to use some of CS 4's Extended features.
Several photographer associations have arranged a discount on Adobe pricing for their members.
Among many, many other hardware, software, general business, travel, car rental, discounts and other perks like NAPP's new online Photoshop/Lightroom training, NAPP members get 15% off Adobe's prices, including upgrade pricing. So we only pay $169.15 to upgrade from CS 5 to CS 6.
Photoshop User | Learn Adobe Photoshop, Lightroom and Digital Photography | NAPP