I recently got to demo a front stairway ceramic tile install. It had been in place for about to years and repaired once already. The steps were thinset and relatively all right except for a couple of areas that were holding moisture and got frost heaved. The side walls were another story. They had been installed with mastic under 12x12 porcelain tile. After ten years the stuff was still like goop and I was able to peel off the tile by hand and hose off the mastic.

Mastic has it's place: in dry applications like a back splash and with ceramic tile smaller than 6x6.

I was tiling new houses until about a year ago and mastic is all we used for tubs and showers (except the shower floor). No good i guess, I doubt they're falling apart this soon but I'd use thinset now for a tub or shower

If you want a mastic alternative, try one of the following two products:

Flextile 66 Lightweight Polymer Modified Mortar
Laticrete 255 Multimax

I've used both products and they both work well - you get the benefits of a thinset without the problems that can come with mastic use in showers ... and no, I'm NOT saying never use mastic, it does have it's place.

I swear by the Flexlite 66 anymore. Last year I did a shower - walls and ceiling - in 16"x 16" Granite with Flexlite. Mix it up, let it slake for a bit, mix it again and off you go. It's sticky and it sets up hard and fast. With lighter ceramics, sometimes you can grout it the same day.

I know that at one time everyone used mastic for walls. Some guys still do and swear that they have never had a problem. I used to, but now I simply have more confidence in a good modified thin-set mortar. Given the choice, the only place that I will use a mastic based product is on a backsplash or for putting in tile baseboard.

Mastic in the pail marked "Ceramic Wall and Floor Tile Adhesive" on the floor has always been a mistake. The Masitc needs air to dry, can take months to fully cure, and behind a 12"x 12" tile never gets air so it never cures. I have never heard of a positive experience where someone tried mastic on a floor.

Watch those "premixed thin-set" products too. They are often a mastic based product. I'm sure they're fine in some applications, but read the pail before you use it. They often say that they are not for use in bathrooms or in areas exposed to moisture.