I bought these for my Nintendo DSi XL, and I found them the easiest to fit of all the screen protectors I've had. They both went on first time, nice and square, and with NO BUBBLES. This may be because of the practise I've had, but this is the only one that's gone down without the dreaded lifted corner (which gathers dust under it until it looks disgusting).
When fitting any screen protetor you need to have a few tools to hand BEFORE YOU START. Once you've begun, if you have to go away and get something you need, the chances are you'll start getting dust on the back of the film, and that's it game over. So, set yourself up in a well lit area with:
1) A lint free cloth. Get one of those microfiber glasses cleaning cloths from an optician's.
2) A length of sellotape (or similar), to pick up individual specks of dust from the screen, or any that manage to stick to the underside of the screen protector
3) A small pocket knife with a pointed blade (the small blade on a Swiss Army Knife is ideal), to pick up the corner of the screen protector so you can remove it if you need to try again for any reason.
4) A credit card, to smooth the screen protector down as you apply it, minimising any bubbles.
Once fitted, these screen protectors have a mirror finish when the DSi is switched off. This mirror effect is said to disappear when the machine is on and the screens are lit up, but if you have the backlight set to low, or there are dark areas on the game screens (e.g. the black squares on a Crossword puzzle), there is still quite a bit of reflection.
I wouldn't say to avoid these FoliX screen protectors for that reason, because you can get round this to a certain extent by turning the backlight up, and you probably wouldn't notice the reflections in a fast-moving game (e.g. Sonic Rush).
The FoliX FX-MIRROR screen protector are very good in every other way, and although I've only just started using them, they do seem to be very scratch-resistant.
This screen protector is brilliant. I have only tried to fit one other screen protector which landed in the bin. I was doubtful I would be able to get these on. It comes with very clear written and illustrated instructions in several languages. I still can't believe I managed to get this on with no bubbles first time. I love the mirrored effect which disappears when the DS is switched on not affecting the performance at all. Well recommeded.