Maniac
Apolyton Sage
Why less than artillery? That doesn't make much sense in play terms. Artillery units can fire once a turn, every turn, in addition to being able to defend their tile. Missiles are completely used up after one attack, and they cost the same as a mass driver, I believe.
Disclaimer: I've never used a single missile in Civ4, so I'm not exactly an expert on the best use of missiles.
Anyway, the reason I added the Thermobaric Missiles was for a niche use: blowing up Fungal Towers, stacks of native life (on fungus), and perhaps Bunkers. (Making locusts extra vulnerable to them might also be a good idea given WarKirby's experiences) My fear is that if missiles are made a very powerful unit, they will, instead of niche, become standard equipment and be used every time to soften up a stack before moving in with the rest of the troops. I fear that might remove a lot of the combat tactics from Planetfall, and just make the victor whoever can throw the most missiles at his opponent from a safe distance. Again, I have no experience with missiles, so I don't know if those fears are grounded.
So I'd like to make thermobaric missiles worthwhile, but only for those couple niche uses mentioned.
Although on that subject, artillery bombardments are a bit too powerful, I think. Especially from bunkers.
In what context did you find this? That is, what did you strike at, or what units of yours did the AI strike? Just asking because there is more than one way to deal with this: weaken their damage, or improve the counters in quality or quantity, or improve the AI if they just don't use the available counters effectively.
Perhaps it could be tweaked so they'd be no more or less efficient than windmills in flat/rainy areas, to create a fun aesthetic choice between the two.
"A designer knows he has achieved perfection not when there is etc etc..."

Having two things do exactly the same thing doesn't sound appealing to me.
1. Interceptors.
Do you feel there is something in Planetfall which makes air power more powerful here than in vanilla Civ4, or do you feel the problem exists there too?
Several possible solutions here too of course, depending on what exactly the problem is. Eg are air units too powerful, or does the AI just not use the available counters? = AAA and their own air units.
One option is to reduce the maximum damage they can do, at least for the first available air unit. Make them something like the vanilla Zeppelin.
I really don't think they should be able to instantly rebase across the world. I'd say there should be a limit on their range, based on their operational range.
I was actually planning this for some time, but then I reconsidered for the following reasons:
1) By the time you get air units, it also becomes possible to instantaneuosly transport artillery units with psi gates. So I wondered if air units' rebasing ability was really that unbalanced compared to other benefits of a similar tech level. Hell, because of psi gates, limiting the rebase range could make the slower than artillery!
2) Limiting their rebase range could make them too similar to artillery units. Now you have the difference "air units = very mobile, but do less damage than artillery; artillery: less mobile than air, but if you can get them on the right spot they can do much more damage."