If you're getting a knife mainly for the corkscrew, you'd be better off with a waiter's friend/sommelier's tool (the kind with a lever and a little blade). The Laguioles here:
http://www.cutlerytogo.com/lagcor.html are very nice, but you're better off with a $5 one from the liquor store and spend the good $$'s on a nice knife. Just make sure the screw isn't one with a solid shaft, they split the corks. I've worked in many bars, some serving very expensive wines that you don't want to screw up and a lot of the guys I worked with would spend £50-£60 on nice waiter's friends, but the best I ever used was a £4 branded plastic job from Oddbins. - just make sure you get one with a nice comfy surface to pull up on. The only reasons I'd spend a little more are for a teflon coated screw, which makes it a lot less work to get the new plastic corks off after you've pulled them, and these
Pulltaps have two step levers that can help with long and/or weak corks and also have pretty decent blades. I have one of these, but prefer the oddbins cheapie in a busy bar - it has a trick foil cutter which saves time.
I have a Laguiole with a 4" blade and corkscrew, but the screw is short and narrow and you don't want to use your main blade to cut foil against glass coz you'll blunt it. The SAK screw is a bit better but still needs muscle. Short screws tend to leave a bottom portion of the cork in the bottle, and narrow ones are more likely to pull out a 'core' of cork or split it.
If you use one without a lever (eg. SAK), put the corkscrew in as straight and central as possible (see below). Hold the bottle by the neck in your weak hand and clamp it between your thighs. You can also kneel down. Get your shoulder right over the cork and use your shoulder and arm to pull straight up, without levering against the sides of the neck. It takes a big pull, but then it pops out easy.
Now for the classy way:
If you use one with a lever, cut around the foil using the blunt little blade by resting the edge toward you on the little step that most bottles have, pressing your thumb opposite the blade on the other side of the neck and turning the bottle. screw it into the cork, straight and centered (to do this, I start with the opener tilted over so that the tip of the screw points straight down - then after its in 1/4" I tilt it upright and turn it with my hand placed right behind the screw so it doesn't wobble). In theory, you want the screw to go as far down the cork as possible without penetrating the bottom (aviods bits falling in). Next, put the edge of the lever against the lip of the bottle (I'm just going to do one now to make sure I describe it right

) the tip of it should fit under the base of your index finger while you hold the neck of the bottle in a natural grip, the lever should be at the 12 o'clock position and the other end should be tilted downward slightly at the 6 o'clock. Lift the other end while keeping pressure on the lever with your weak hand. If you just push naturallly, you will tilt the cork away from you, bending it against the neck and risking breakage, so LIFT up with slight backward (ie. toward you) presuure to lift the cork straight out with a pop. It amazes me how few people know how to do this properly, but it works most of the time even with dry, crumbly corks. I reckon I only mess up somewhere around 1 in 1000.
Here's how to salvage when you mess up, or impress/embarass someone else who has:
Usual scenario: you're at a party and someone hand you a bottle they just tried to open with a spoon. You're thirsty and so is the beauty you just met, but you're stuck with a bottle with a 1/4 to 1/2 a dry, crumbly cork and you can't get the screw into the cork without pushing it into the wine, which makes it a PITA to pour, never mind picking cork out of your teeth.
Worst case scenario, you're standing at the table of Lord whatdymacallit who's really picky and has just ordered a £400 bottle of wine that's so old its probably off already, not that he'd be able to tell but he won't be happy if he gets a bit of cork in it and you boss won't be happy if you put it on wastage.
What do you do?
Easy. blow out the loose bits of cork from the top of what remains, and intstead of turning the screw straight into the cork (hich will push it down) slip it down the side, between the glass and cork so that you're not pushing the cork down into the wine, until it pops out the bottom, then *slowly* and *gently* pull the cork up through the neck (should be easier due to less friction). Voila!