We have been sharing some good ideas, and things that might help others with less experience so I am going to ramble on a little more. And I agree that getting parts off of BuildTak can be difficult.
My findings are that there is not a one size fits all ideal with print surfaces and a lot depends on what plastic you are using, what you are trying to print, and the quality level that you are trying to reach. I will also say that I have 7 different print surfaces for my FT-5, some of which will probable never be used again because they were failed ideas and some that I will use a lot. One of the failed ideas was taking an aluminum plate and sanding it with 60 grit paper to make a textured finish and then using hair spray on it. If an idea sounds stupid try it any way - you don't have to tell anyone what you tried. Using these printers is as much an art as anything creative - you need to learn what works for you.
Most of my prints are made with ABS and in general I have excellent luck using window glass and hair spray and yes I have the 12x12 mirror tiles but think things hold a little better on the window glass (and I have no idea why). Recently I have been print lots of cable chain parts, I need 2 1000mm chains for my Laser Cutter. The problem with the cable chain parts is there is a very small surface that is on the bed and I have had a very hard time getting them to stay on glass, and also had problems with PEI with these small parts. Some times it would work and other times it would not. That is why I got the BuildTak because I knew that the parts would hold much better - and they did. I figured that I would rather work to get the parts off than to deal with lots of failures because they would not stick, as a side note I know that I said yesterday that it worked at 60C and it does but I found it works much better at 80C so that is what I am using. As a side benefit you can use a much lower bed temperature than glass which can be useful for anyone having problems getting their bed over 80C. It is better to work to get a part off the printing surface than not being able to print ABS at all. And the BuildTak has worked out great for these small parts and have not lost a single one on this surface.
We can learn a lot working with different surfaces and seeing what works and what does not. I have lost count of the number of calibration cubes I have printed working out what surfaces work and at what temperatures for different plastic. You do need failures to make progress and learn. Once you have some good experience when something does not work you have a better knowledge base to work on fixes.
So I hope that this rambling is useful for someone and if there are questions/problems always ask because someone can help.