DeBussy: How long can I use such a kit? Do I switch freely between them, or can the provided weapon-skills only be used once - and then the skills automaticaly reset to my normal weapon?
Jon Peters: You toggle them, and then that is just the weapon set you are using until you decide that you are done using it. They are balanced around the idea that you switch them situationally. Some of them have different pacing to them. So for instance the med kit: you drop a lot of med packs on the ground and if you just send them all out quickly then you are left with not much to do. And so you can switch back and start doing more offensive things.
For example with the grenade kit. The regular grenade throw has no recharge on it and the other grenade throws are pretty low on their recharge as well. So if you want, you could just be a grenade engineer. I have people seen do this - just running around throwing grenades. And that's just what they want to do. And they rarely switch back to their rifles and pistols.
One other thing about the kits - and what also applies to some of the other utility skills: The Engineer has a tool belt. These are his special F1 to F4 skills located above the normal skillbar. Any of the backpack kits have a skill that you automatically get on you tool belt when you put them on your bar. The best example is the mine kit. If you put a mine kit on your bar and you activate it skills 1 trough 5 are replaced by mines. And as you throw them out they turn into separate detonate mine skills. So you could detonate each mine individually. But we wanted the mine kit to have more synergy with other weapons and other kits as well. So when you switched away from it you lost the ability to detonate the mines. So what the tool belt does for the mind kit is: it puts a skill above your bar that you can use regardless of whether or not you are in the mind kit, that detonates every mine.