Maddin is mostly right ...
trak5 glass may be ugly ... but it is flixable ... there is no situation where it will look horridly out of place ...
(afterslime uses this)
cube map glass has to be custom made for the environment that it is put in
also ... It only works on small glass patches because of the way a cube map works ... a large area will not look right while you are moving ...
(redplanet and solarium use this)
The prettiest glass uses reflections ... however this drains your performance is used improperly ... (the old nexus map starship uses this glass properly)
The other thing that will make glass look great is distortion ... but this only works right with the reflection glass (otherwise you get this horrid z-fighting) ...
so ... glass is no easy matter ...