goldsax,
A poor fitting liner is a good indicator of a pool that is out of whack, so if you intend to do this project yourself be very careful when measuring.
As for what to look for when buying a liner. Firstly lets start with blatant fraud that's going around on the internet. Many online liner retailers are calling their liners "25 gauge" when they are in fact 20mils thick. Whenever you see the term "gauge" for vinyl that should be a red flag for you because vinyl thickness is not measured in gauge, it's measured in mils. If they say "gauge" they can call it whatever they want "25 gauge", "30 gauge", "50 gauge". (I've toyed around with the idea of calling our liners 1,000 gauge vinyl, just for kicks)
Got that out of the way..
Here is my personal philosophy on liner thickness: 28mil wall material with a 20mil bottom is best unless your pool bottom is a bit rough, then 28/28 is the way to go. Here's why: most liners fail above the waterline because the wall material has disproportionately stretched to the point of degrading. This happens because, upon installation, gravity is pulling on the liner. At the bottom of the liner there is zero force pulling on it. Half way up the the liner half of the weight of the liner is applied as force and all the way at the top 100% of the liner weight is being applied as force. Now, once you add water to it the weight of the water starts to pull on it even with a good vacuum on it. So, if your liner's bottom material is thiner it will allow for more proportionate stretching of the liner.
If you go with all 28 mil material you will be putting that much more weight on the liner which kind of defeats the purpose. Although, if, for instance, you have a hard concrete bottom to your pool, you may want to go with the heavier mil liner because the little grains of sand/pebbles that may end up under your liner upon installation may eventually wear a hole in it.
...this is getting long winded, isn't it.
Alright, last paragraph.
When you are shopping around for a liner beware of "regrind" or recycled vinyl. This type of vinyl is reprocessed and often has impurities in it. Make sure to get it in writing that the liner you are being sold is not made from recycled material. Some of these "great" online deals are people selling recycled vinyl liners.
Dang... one more paragraph then I'm going home...
I'm not a fan of vinyl from overseas (china). It is very, very inexpensive material, but china doesn't have a very good track record for quality control so I opt not to use it in my liners. If they put lead in my kids' toys, what are they putting in my vinyl?
In closing (geesh this post got long) if your pool is measured correctly and your liner is installed correctly and you maintain your chemistry, and you go with a 28mil wall with a 20mil bottom, 15 years is not an unrealistic expectation.
Good luck.
