Which F skill?
Each one has a use, and you will more than likely be using all 4 of them.
F1 is your DPS shatter
F2 drops large amounts of confusion, and if traited, gives you retaliation
F3 is a stun, good for interrupts if you have Illusionary Persona traited
F4 to dodge everything 1s per illusion
Sybol – Healing Bunker Charr Mesmer (80)
Dresdon Honorclaw – Zerk All-Ranged Charr Warrior | Hawke Fullmoon – Melee Ranger
It does depend on your build.
Most mesmers will however use F1 quite often.
In shatter builds off course as often as possible, other builds would use it to finish off the enemy with a fast shatter.
F2 is also used in shatter builds when F1 is on cooldown, or for conditionbuilds for spreading mass confusion, or builds that want to make use of the retaliation it grants with the confusing cry trait.
F3 is probably the least used shatter, usually its used as a fast interrupt with no illusions up trough the use of the illusionary persona trait.
F4 is your get out of jail card, can give you up to 4 seconds of invul, it is on a long cooldown though, 45/60s.
Luna Solares – Mesmer
F3 is a stun, good for interrupts if you have Illusionary Persona traited
Just to clarify this since the OP is learning, F3 is a daze. (They can still move, they just can’t use skills.)
Also, a while back I saw a thread that was talking about keybindings. One of the devs (don’t remember who, but he said he was really tall) noted that he remapped his keys. F1 through F4 became Ctrl+1 through Ctrl+4. I tried it out and it really increased my effectiveness as a Mesmer.
Getting used to new keybindings is difficult. To ease the learning curve, I started by only using Mind Shatter (the Ctrl+1 skill) with this setup since it’s the most common shatter. The other three shatters quickly fell into place after that.
Dragonbrand
Oh yeah, ever since the second beta weekend (when they enabled key mapping) shatters have been ctrl 1-4. The reason for this is that the f keys are farther away than any other key you normally press. Using ctrl as a modifier is natural, and adding 1-4 to it is natural as well.
For the same reason, every skill past 5 is bound to a key near wasd, for me going q e r t x from heal to elite.