The spy will run slower than normal if disguised as slow classes but wont run faster than spys normal speed when disguided as fast classes.

I saw this video at the start of my spy career and learned alot then but now... There's nothing to learn.

Jono, I've got a self-writen guide somewhere in this section of the forums, have a look at it if you haven't

Just a few tips from me as a Spy when using some of the weapons and devices:

Revolver:
1) When you notice a player as an enemy spy and he notices you as an enemy spy, the best thing to do is to pull out your revolver and start shooting him. He will soon follow, but at least you get the first hit. Never leave him to run away, always try to kill them if you see them, only when you are in a safe place, because they may come back and kill you with the Revolver or even worse the butterfly knife which is very humiliating when your a spy.
2) If you stab someone and the back stab doesn't work then don't just run away. Instead, run away and start shooting them. Your out of your stealth and disguise and their going to start sweeping the area for when you cloak away. Hopefully from your knife weakening him you can finish him off with a critical strike from your revolver.
3) If you see your team killing an enemy, finish him off with the Revolver. It is a very powerful weapon with a very big critical strike.

Butterfly Knife:
1) The knife has an instant kill from the back, or from the side, or from above. Always attack from the 180 degree of the back, anything more like from the front or just a bit more forward from the side will result in just damage being taken with no instant kill. If you can, attack from up high.(Doesn't matter which side you are attacking from when you are up high, just make sure you hit the top of their head)
2) Make sure that when you are behind someone and you press to click, give the knife sometime to actually hit the enemy. I and many others have made careless mistakes of just clicking on someones back and just walking away straight after. To prevent this, wait a second after clicking so that the knife can register the backstab. If the person is running, try to run as fast behind them and then click, also wait 1 second for the backstab to register.
3) Sometimes the knife will not do a backstab and instead will just do a stab even when you are behind the enemy. The stab is a lot more quicker then the backstab, so don't worry about it not instant killing the enemy from behind because it will.

Sapper:
1) When you are sapping sentry's, try to not stay in the area for long, for all you know there could be a Pyro right around the corner.
2) When sapping engineers buildings, sap ALL the engineers stuff around him. This is helpful for when he is finished un-sapping the sentry and then runs to his other stuff, you can sneak in another sapper on the sentry and maybe even destroy it.
3) You can put on multiple sappers at any time.

Disguise & Cloak:
1) Make sure that before running out into the battleground that you are disguised before cloaking and that you let the cloak cover your full hand before running out. This will insure that you are fully cloaked and disguised.
2) When choosing characters to disguise into:
Scout - Bad choice, people can recognize you a mile away that you are spy because you don't run as fast as the Scout when you are disguised as one.
Sniper - Okay choice, just make sure that you are in a Snipering type position so that it looks like you are trying to snipe someone.
Soldier - Good choice, however just try to avoid walking into people and people walking into you as you have a slow movement speed.
Heavy - Good choice, again watch out for the movement speed. You may also be ubered when disguised as a Heavy. Just play along and then get to backstabbing when it is finished.
Demoman - Good choice, again you may be ubered.
Pyro - Good choice, again you may be ubered.
Engineer - Okay choice, just make sure you are surrounded by sentry's or pretending to build something or look like you are running to help some sentry's be built.
Medic - Bad choice, people will expect you to heal, and if your not healing. BANG. If you have no uber charged, they will probably wonder again why your not healing. BANG.

Pretty much it. Thanks.

Actually scout can be quite usefull, just dont use it in open spaces where the speed will stick out.

Bad choices, definately. See my guide for explanations

Here's another handy tip as a spy!

You've probably seen this already but I laughed so much I though it only fair to share.

I've seen that before and I laughed and laughed .

I think i disagree with that, and i never do it, for the following reason:

You run fully cloaked behind enemy line as a pyro. Someone bumps into you, and you (and the shape of your disguise) is revealed. People will know that there is a spy AND will double-check pyros (in this case). If you run around as spy, they will simply know that you are a spy...

Kay, thinking about it, if people double-check for the pyro and you change your disguise while cloaked, you will be less likely to be spotted. You could use that for your own advantage: run cloaked in a useless disguise, one that you will not need anyway in the current situation. If they reveal you, change your disguise and you are a bit safer...

If you bump into someone and they look for you the first thing to do is to get the hell out of there through the nearest exit, simple as.

it can be good to go as a soldier. people are so used to spies being disguised as pyro's, engies etc that an unconventional disguise can make a difference.
It al depends on the situation...

one thing that I also find helpfull is to walk backwards when entering a base.
you know, normally you always look at the direction the enemy could be coming from. so if doing this -even for a short while- can make you look more 'real' (more than just walking la-di-da straight to the sentry)