There are many methods of delivering the power enhancement of NOS for an automotive application and how well, reliable, drivable the vehicle ends up will depend upon how well the installation is thought through and implemented.
Adding 100HP is not a problem but how well your tyres, transmission etc will cope with the extra power will vary. Also - longevity of the motor will depend upon being realistic with your expectations. Trying to stick 100HP on a well built small block is not likely to present any problems but adding it to a tired, over-the-hill motor that is built from lower spec components will likely make it short lived. A motor built with quality billet parts and forged pistons etc will allow a greater headroom for power adding.
In terms of use - well you can reckon on just under 1LB of NO2 used per 100HP / 10 seconds. SO - a 10LB bottle will give 100HP for 100 seconds - yes? - well no, because as you use it up, the bottle pressure will eventually start to drop and the resulting mixture will become FAT or rich so you will never properly empty a bottle during normal use. Bear in mind the size of the bottle too, a 10LB bottle is about the size of a 12L diving tank, so not exactly small.
TIMING is ALL important and it should be retarded for the hit of NOS, with a general rule of thumb being 1.5-2 degrees per 50HP added. So a 100HP hit would require a 4 degree retard too. Colder plugs may well be required too to cope with the increased cylinder temps so be prepared to fit them.
Also - consider how the power delivery is required. A friend of mine added a 150hp shot of NOS to his 350ci Chevy and whilst the motor easily held the power, as an 'instant hit', the tyres would not and despite being slicks, they lit up every time the system fired. In these instances, a small power controller can be added to ramp up the power, so that for example, the initial hit is 25hp, or 40hp which then ramps up to the max required over a period of time that allows the tyres to cope, say a second perhaps. This time will vary depending on weather conditions, road/track surface adhesion (grip) etc etc and only experience will allow you to dial that in .
Yes, the system can be activated by a switch at full throttle, in fact it should only be allowed to fire when the throttle is fully open as that is when max power would be required. It may also be prudent to avoid activating it below certain rpm levels, so as not to load things up too much by expecting too much too early.
Again, another friend has a blown motor, to which initially 100HP was added. When the NOS fired, the measured torque on the dyno jumped to over 850ft lbs at 3000rpm!! One advantage of NOS on a blown motor is the cooling effect, which will generally give an even greater HP advantage over the normal calculated/expected increase.
One pitfall is chasing times/performance and any system should be used to a pre-determined limit/level. The temptation to 'turn it up' is ever there but just remember how much you are asking of your motor and keep it sensible.
All of this really only breaks the surface of NOS (NO2) and in reality, anyone thinking of installing such a system needs to carefully consider ALL consequences / requirements and if unsure at all, either don't, or go to a professional tuner/shop that has a good reputation for such installs.
Do also consider the legal / insurance requirements for such installs too if you are intending to use them on the road.