yes, it can be a pain in that if you are doing small water changes you need to use TINY amounts, dont be tempted to use too much, its a very concentrated solution, BUT its not particularly dangerous in overdose apparently and it is supposed to be used at higher strengths if you need to detoxify nitrite or have very high ammonia levels, its all explained on the bottle.
With this, can you only do it to the total amount of new water you put in? Or as the total amount of the tank?
the idea is that you dose ONLY for the new water going in, i am looking at the bottle now, therefore if you are putting in 20 litres of water as part of a water change, you would only need 0.5 ml of the Seachem Prime (although getting that small dose is tricky and i would say 1-2 ml wouldnt be a problem)
If someone has just one small tank it might be best to stick with the more mainstream conditioners (just for ease of dosing), but for larger or more regular water changes i would definitly recommend PRIME, its also one of the best anyway (not just in value), i had a good link comparing all the "conditioners" but cant find it now.
If your tank had a nitrite or ammonia problem, then you can follow the guidelines and dose the WHOLE tank with it.
If you speak to anyone who REALLY knows about the science of water conditioners (i.e. dealing with ammonia, chlorine and chloramine as a package) they would probably reccomend SEACHEM PRIME