I have heard people recommend borosilicate glass on top of the heat bed. Is there any reason to use it over toughened glass?
I just spoke with a supplier and borosilicate is at least 2 times the price of toughened, quoted 50 quid for a 275x300mm (my build area) piece. +15 postage. The supplier told me toughened glass goes up to 300C so this should be suitable also?

Another issue I am concerned about is fixing the sheet down. Do the bulldog clips hold it very firmly? It seems like a bit of a bodge. I know I am being presumptuous here but maybe people dont even realize minor print defect are maybe being caused by the glass plate not being firmly fixed, ie with screws?

I find that 2mm standard glass is fine even up to 130C I have even put abs juice at 100C with out problems.

Regular window glass works fine as long as it's evenly heated using something like the Prusa PCB heat bed.

The clips hold it firmly enough to not slip.

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[www.hydraraptor.blogspot.com]

Many recommend borosilicate. I have always used just normal window glass, according to my supplier everything they have is "float glass" which is very flat.

I used the same sheets for over 6 months with no issue. Then I got a bit larger glass and countersunk my bed screws (aluminum resistor bed) and I have had a couple sheets crack right at the screws. I think either I did not deburr the holes exactly flat or the difference in heat transfer from the 2 types of metal (aluminum bed and stainless screws) is causing the cracks. Like others have said make sure there is even heat.

I use ~3mm glass sheets. I do have some boro that I am about to try though.

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I am using replacement glass for 8x10 picture frames, and cutting it down to 8x8. I have seen great results with Kapton tape and 110 prusa MK2 heat bed. The clips hold the glass down perfect for me.

Some more FYI - I'm currently using a cheap light mirror (maybe 1mm thick) which I cut to 200x200mm, on an aluminium bed heated by resistors. My bed bolts are countersunk to give a flat top surface. Bulldog clips hold the mirror. I've no concerns about the clips and slippage. The mirror has little mass and there is no drag from the print head on it. I'm only printing PLA at present so the mirror is not subject to the same heat as it might if printing ABS.

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[numbersixreprap.blogspot.com]