casa zen is one of the few places in mal pais that offers a really solid "middle-ground" in terms of price and atmosphere. the single family ownership have made a point of keeping prices down while also making sure the place is well staffed, clean, and comfortable -- with some style. you're experiencing a labor of love here, run by people who have repatriated to costa rica to stay -- not some real estate developer's detached idea of a boutique hotel.
you could pay 2x + the price elsewhere easily and NOT get 2x the value. there's not much middle ground around in the immediate vicinity-- prices nearby jump to $70/$120 night for private cabinas at nearby itauna and tropico latino during high season, and the budget options nearby are... uh... well, muy rustico. casa zen still offers hostel-style bunks in addition to private rooms (shared bath) and an apartment with private bath + kitchen that can optionally connect to additional bedrooms for larger groups. there's also a communal outdoor (covered) kitchen downstairs for everyone else.
and, then there's the restaurant. the food's excellent, thai dishes for dinner and typical/traditional breakfast and lunches.
its true, due to the surfer/backpacker presence you may encounter some people hanging out at night and they'll be audible if you're turning in early to sleep and get a jump on the surf in the AM, but i just spent a month there, and in fact the staff DID make a point of asking folks to tone it down after 9pm or so on the few occasions it was called for, and a few semi-polite words like "hey dudes, keep it down", or "por favor, amigos" also does the trick. people who stay here aren't rowdy, they're just happy, and its a pretty tranquil place in general.
for me, a major plus at casa zen is that it is centrally located but still away from most of the development in the area. you don't have to walk on the main road to get to the beach (its beachside), there's a fully stocked market a block up and over, and its equidistant from both playa carmen (beginner surf mostly, apart from the outside peaks near the reefs) and santa theresa (excellent and occasionally heavy beachbreak waves for experienced surfers when there's a swell -- and if you want to score it good, just get up EARLY instead of drinking that last shot of tequila the night before... you'll get glassy and quite possibly hollow waves in return, with only a few other people in sight. you want a mid-tide usually, coming up... unless the swell is small, in which case, plan around the low tides... this place yields tubes.... FREQUENTLY... more often right, but also left on a few peaks).
the staff here is excellent, all the way around. it isn't a barcelo, mind you. if you're looking for a turn-down and a chocolate on your pillow, you're in the wrong place. but the people here are all nice and good at what they do. the yoga classes are both solid yet non-intimidating as are the spa treatments & massage.
there's also internet access here (2 newish computers with a printer), and since it isn't shared with 10 other people trying to use it at the same time its pretty fast -- so if you need to check in with the "fake world" on a regular basis, you don't have to hike to some place up the road. its all right here.
if you come here, you're going to make friends, and want to come back.