I main an I/L mage and I can tell you it's quite fun to play, and some of the skills are pretty useful, particularly freezing breath.
With freezing breath you actually don't need to hold down the skill, especially if you are soloing, because the freeze still lasts the full duration - all you have to do is make sure the enemy is frozen. For Vellum you use freezing breath when he spits those one-hit fireballs, that way you freeze him when he is above ground. For RA I recommend this round of attacks - freezing breath to freeze, followed by ice age (5th job), blizzard, frozen orb and then hold down lightning orb. With my current range I can kill RA bosses with just one of these rounds of attacks
I wouldn't recommend Lumi, especially after playing I/L for so long, because it just doesn't play as well as I/L
F/P I have heard is better than I/L for damage but F/P is mostly DoT, and to get huge damage you have to set it up, which may be end up being the same as just spamming one skill. There is also a lot of debate as to I/L and F/P caps which I am not too sure about
