Very harsh. Kind of good, too, but silly if you want to cast Fly 5 times, one per party member, that should be supported by the game one way or another. My vote is the spell recipient must concentrate, that way you can still buff multiple people, but each one needs to maintain their own buff stack and concentration.
I am under the impression that the designers might want to prevent that to happen: they want to prevent stacking buffs on each PC and they want to prevent or at least restrict mass effects that can alter the whole course of the adventure (although
teleport spells are still mass-affecting at the moment).
On one hand, I think mass effects are valid tactics so it is not completely fair that the game specifically prevents them.
On the other hand, I really hate seeing the whole party of PC fly... I hate combat with flying characters because it's usually a nightmare to run, and even if it had simple rules it would still look silly. I don't think it adds
anything interesting compared to fighting on the ground. OTOH a combat where some
monsters are flying and the PCs are not is indeed interesting and may worth the DM's effort.
The other use of fly that I hate is the overland travel. I don't care at what level it happens, the look & feel of it will always be plain lame to me. It's not that I am against the PCs findind a way to bypass distances and terrain, it's just that when this is done by casting one spell and go, it feels like it wasn't earned at all... It's an entirely different matter if you have to find ad-hoc means for that, be it capturing and taming flying creatures to ride, or obtaining a precious flying carpet.
As you can see, I am torn apart between wanting to allow the mass tactic for groups that are fine with it, and not wanting to have boring situations in our game.
You could also make Fly have a duration that says you only fly when you are concentrating, but the duration endures, so that casting another spell (say, an offensive one) requires you to land temporarily. But that very much reminds me of 4e and I want Fly to be good again.
This could be fine in my games if the whole duration is also max 1 minute or so. I would be ok with a
Fly spell in the game that can be used to reach a difficult spot or to bypass a
single terrain obstacle. Something that would be basically an improved version of the
Jump and
Levitate spell. It's when used in combat or for overland flight that it becomes lame for my tastes.
Mass Flight could be a higher-level spell or a metamagic effect that does the same deal as regular Fly but for more people (so that while you or the target concentrates or maintains focus, the entire party can fly).
I was also thinking of a metamagic feat, there was something like that in 3ed, or even better a special feature of a single wizard tradition. It could fit with a battle mage that is more focused on group tactics than mere blast spells.
