Combats in Felltower very, very often feature Missiles spells. Probably 50/50 regular and explosive. Combats in Felltower often feature table discussion about how Missile spells are underpowered, hard to hit with, too easy to defend against, not cost-effective, and really unfair to wizards because they should be able to do some damage-doing effect every turn.
I'm totally serious about both. We see a lot of Missile spell use. It's almost every combat.
We do have some or all of that discussion of Missile spells, even as people are using them.
It's odd but true.
I think the "Missile spells are weak and not cost-effective" opinion has a lot of validity to it. I don't think it's true in Felltower.
Underpowered: Missile spells do 1d base damage per 1 point of energy, or per 2 points if explosive. A 3d spell does about as much damage as a front-line fighter's hit, but often less. It's a little weak compared to fighters. In DF Felltower, this is the same . . . but I feel like it's mitigated by two factors - the ability to hit multiple targets or single targets with the same spell (since Explosive spells aren't a different spell), and that they are often coupled with an effect melee weapons don't get - electrical stunning, armor divisors, burning, or even high-damage impaling. It's not a 1:1 matchup but I don't think they should be, nevermind need to be for fairness.
Speaking of underpowered, the rules for blast effects for explosives in plain GURPS (and even DFRPG) are a bit underwhelming. DF Felltower makes all explosive spells area 3 (so diameter 5). Damage in the center hex is full, in the next ring out it's 2/3rds, the third ring 1/3rd. That makes big explosive spells less fratricidal and smaller ones have more utility in hitting an area. It's also faster to play.
Hard to Hit With: Hitting with a Missile spell takes an attack roll, which is a DX-based skill, and wizards aren't always good with those. But DF takes care of that in two ways. One, even wizards routinely have a 14 or 15 skill in it. And by the book, you can buy the Psychic Guidance perk to let you use your spell skill instead as your attack skill. DF Felltower uses Mystic Guidance, instead, which rather gives you +1 per level of your spellcasting talent as a plus to your attack roll. I just find that a better solution, but either way, in DF Felltower it's only hard to hit with missiles you don't put some of your available spellcasting perks into improving with.
Too Easy to Defend Against: This one I just disagree with. They're not harder or easier to deal with than any other missile, and the explosive ones are usually thrown to hit with the blast effect so it's hard to avoid damage altogether. Just no to this one.
Not Cost Effective: In standard GURPS Magic, it's 1 for 1d and 2 for 1d explosive. You can save 1+ energy per spell for a high effective skill. A Magery 6 wizard can throw an 18d spell for 18 or 36 points . . . with skill 15 that's 17 and 35, with skill 20 that's 16 and 34. In DF Felltower, it's 1 for 1d or 2 for 1d explosive as standard, but you can subtract for your effective skill each turn. That 18d spell costs 15 or 33 at skill 15, 12 and 30 at skill 20. Still expensive, but much more doable. A 9d - the cap for Magery 3, and still a very nasty attack - would cost 6 and 15 or 3 and 12 for skill 15 or 20, respectively.
The damage you can cause to a packed formation of foes with effectively 9d/6d/3d attacks, especially with a Fireball (3+ ignites fires, 10+ ignites everything on a target, and average damage will do 10+ at all three bands), is terrific. It's not going to cause all of your foes to char and die . . . but it can make all of them combat-ineffective. Not bad for 12 or 15 points, given that most save-or-die type spells cost around 10 per target and are resisted.
Can't Do Something Every Turn: Well, fair enough. It takes 3 seconds to pump up a spell to the most powerful level, and 1 second to throw. So they're 4 second spells. Seems a bit much . . . or does it? Personally I disagree with this one. I don't think you need to be able to do damage every secod in order to be combat-effective. Wizards are critical for combat - ask any foe who confronts Missile Shield or Levitation or Panic spells with no way to counter them. They just aren't pure damage engines. The Heroic Spellcaster advantage from Delvers to Grow can help make them so . . . but I feel like the changes I've made make that less necessary. The author may disagree, I'd have to ask, but I think the many changes we made for DF Felltower make for better Missile spells without needing to add this advantage.
I don't think you need to deal damage every second to matter; I actually think you gain a lot of non-combat ability and combat flexibility by choosing to use magic but can't also effectively deal damage every second.
And I think the constant use of Missile spells in my game says something about the value of them, modified as they are.