My crummy, fluctuating FPS means I can't aim and can't dodge, so I play Heavy, Engineer, and Medic.
Playing heavy gives you a great appreciation for support classes. Without a teleporter, you can spend more time walking than fighting. So when I play engi, I build the teleports first and the sentry last.
This, so much this; my primary classes are Heavy, Medic and Pyro. Playing heavy and medic both teaches you a lot about working with other classes; both are practically useless on their own (okay, not useless, but seriously less then effective), while pyro is great for pushing the need to find alternate paths.
A smart engy with a good teleporter for speedy, sneaky access to the front, a dispenser for those much needed health/ammo refills, and a sentry to hide behind when marauding scouts etc. come past is a godsend, and should be buffed/defended/spychecked as much as can be spared.
I don't understand the appeal of playing Rambo medic. I get killed often enough with my usual cowardly approach. I have had fun with the crossbow. Spamming long range shots into crowded areas gives hilarious results. You're helping the team no matter who you hit!
As Naxza says, it's an effective tactic when you don't have anything better to do; especially scouts. One on one, a blautsauger medic shouldn't have any trouble with a scout or pyro if he sees them coming. Even if surprised, the number of times I've forced a pyro or scout to actually hesitate when the 'soft n squishy' class goes charging (or backpedalling while shooting if v. pyro) while pumping them full of rapid fire hurt is insane; and, because of the health drain of the bs, you have them once they hesitate. Just make sure you learn to aim the syringes (remember gravity, and turn on 'on hit' sounds).
Of course, a single medic charging a whole team is usually a complete waste of time and resources.
EDIT: Ninja'd by Naxza, I was originally referring to his post a couple of pages back, but he makes good points this time around too...