Since my name has been mentioned, I'll jump in. I generally use flaperons and spoilers (spoilerons, as others call them) on all of my planes. At least all of the sport and 3D planes.

Let me dispell a few misconceptions. By and large, you don't lost aileron control when you use spoilers or flaperons. At least, I have never noticed any. I admit that I do like a lot of aileron movement, but I use a lot of expo to control it. Your ailerons will be less effective at a high angle of attack, which both types of control can lead you to. You can also get into a high angle of attack without either spoilers or flaps.

Flaps and flaperons tend to not be very useful to slow you down for landing on a sport plane unless you use a lot of flap, 45 degrees or more. They normally cause you to float on down the runway, way past your intended landing point.

Flaperons are great for turning tighter for more loops at your club funfly or just for messing around.

For landings, especially in high winds and in cross winds, I love spoilers mixed to the throttle. Having both full span ailerons up tends to wash out the whole wing, causing a loss of lift. This lets you descend steeper or get the nose up and slow way down. On a good windy day, You can nearly drop vertical. In a 3D plane, you'd call this an elevator. I use these on every sport or 3D plane.

Right now, I am down to 4 sport planes, an Ultra Stick 60 & 40, a Wild Stik 40 and a Tango (great little plane, by the way). All of these use both flaperons and spoilers.

As for slipping, I rarely do it on a model. I am very familiar with slipping in a full scale. I taught it for landings and simulated flame out patterns while I was a T-37 instructor pilor in the USAF for 4 years. Slipping with an RC model is fine if you have experience and know your plane. If you use a lot of rudder for the slip and also get the nose up to slow the plane, you have just put in snap roll controls. Pop the power in before you release these controls and you can get a snap. I have seen it. It just takes practice and experience. Some guys like it and use it a lot.

All of this brings me to a point I'd like to make. To progress in your RC flying, you need to learn to use all the controls of your plane. You can't say, "I'm going to use ailerons for turning all the time," for example. At harrier speeds, which is normally below stall or control speed, you are holding the plane at a very high angle of attack, increasing power to let the prop blow air across the wings for lift that you wouldn't be getting from forward speed. You are riding on a bubble of air and your ailerons are good for keeping the wings level. If you bank too much, you slide off the bubble and gain speed, so you yaw the plane around with rudder and level the wings with ailerons. If you use a lot of rudder, you may need to cross control the ailerons. What you are doing is using all the controls to guide your plane. If it takes rudder, you use rudder. If it takes power to create lift, you just got yourself a blown airfoil.

Now then, back to flaperons and spoilers and how I set them up.

First, you need to go to a Flaperon wing type (on JR, anyway). After you do this, you will plug your 2 aileron servos into 2 different channels. On JR the right aileron servo goes into channel 2 (ailerons) and the left one goes into channel 6 (flaps). For Futaba and Hitech, I believe it is channels 1 & 6.

After you set a Flaperon wing type, go to Elevator-Flap mix. This is normally a pre-programmed mix on 6 and greater channel radios. Hold up elevator and program in 50% down flap. Hold down elevator and program in 50% up flap. You should set a switch to turn flaperons on and off.

A side effect of setting a Flaperon wing type is you can adjust each side individually for centering (subtrim) and end points.

For spoilers (spoilerons) go to a free mix. On my JR radios, I normally use mix 3 or 4. These are the non-trim, single point, simple mixes.
Set throttle as master and flaps as slave.
Pull the throttle stick to idle and set the mix so both ailerons move up about 15 degrees. As you advance the throttle, the spoilers ought to drop down and be back to the normal aileron position at half throttle.
Set a switch to arm the spoilers. I normally use the Landing Switch.

Take your plane up and fly a normal landing pattern. Note the final glide angle. Next time, arm the spoiler switch. Cut the power to idle right opposite yourself and start turning final. You may need to point the nose down a little, but the plane will assume a steeper glide path. If you are going to touch down a little short, add power. Your plane speeds up a little and the spoilers also drop down some. Both will move you forward and slow the descent. Reduce to idle again and descend to touchdown.

You'll need to play with the spoiler angle to get what you feel comfortable with. At a certain point. you will get to where you are close to running out of up elevator to flare the plane. The max spoiler it the point where the plane is dropping like a rock and you get one pull to full up elevator to avert a hard landing. You have to time it right or it's curtains. I know from experience. A more conservative max spoiler deployment is where you get 2 pulls on the elevator, one where you mis time it and the other to finally touch down.

Spoilers are a ton of fun. Everyone with a computer radio should try them a few times. It feels great to get out on a windy day when someone had to carry your plane out and you get to do multiple touch and goes with a near vertical descent. Try it.