I was wondering how the way the n900 battery is charged affects the speed of this process or the battery life itself.

I personally use the included adapter the n900 came with in the box with my old N95 pin charger as I simply didnt want to unpack unnecessarily (or not?) the new dedicated charger.
The adapter itself gets hot sometimes while charging so i suppose there is some energy lost there.

Can I continue to use the N95 charger with the adapter to charge the n900 or should I unpack the n900 charger from the box and start using it instead? Will there be any benefits like speed of charging or so?

And what about charging via USB from my laptop/PC?

Cheers,
Ralph

As long as the charger supplies the correct voltage (5V) and is capable of providing more power than the N900 is using, then it will charge. The higher the current that the charger can supply, the faster the battery will charge (up to a point). As reference points, USB from a PC is 500mA and the supplied charger is 1200mA.

I've used both the adapter together with an older Nokia adapter (the thin one, not the really old Nokia adapters), but most of the times I either charge using USB (when I'm in class or not at home) or the supplied charger for the N900.
Like Rob1n already said, charging via the supplied adapter is (by far) the fastest method of the three I mentioned. Between using the adapter and USB charging it actually seemed that using an old Nokia charger took more time than charging via USB, but I might be wrong there

I have been using supplied wall charger usually, but sometimes I use USB charge (at work). Few days ago I noticed that when I was using USB charge the phone showed that battery is full, but batlevel applet just showed ~91% (1168mA). I disconnected USB and walked to my car and connected phone to a car charger. After the car charger the batlevel had rised to 96% (1222mA).

I haven't been using that batlevel applet before so I can't tell that is this normal behaviour, but after that I have been using wall charger to my main charger.

Recently I tried using an old ipod wall charger with the supplied USB cable, but sadly that did not work.
So I'm back to charging via USB (at work) or the normal wall charger at home.

I didn't see any differences between those, but I don't actively monitor my battery level except for 'wow, need to charge before I get on the road'.