There are so many stories floating around in the music world about sweep picking. I mean stories that it distorts sound, tosses rhythm out the window and stuff like that. Here is a chance to check out the facts firsthand and I'm sure you'll grab that opportunity with both hands.
Let's begin with a scenario where you're playing a major scale having three notes per string. If you use alternate picking technique you would go down/up/down right through the scale, right? But if you sweep pick, you'll pick down/up/down on the first string followed by DOWN/up/down on the lower string and DOWN/up/down on the lowermost string while ascending the scale. You haven't missed the fact that there will be two consecutive down strokes when strings are changed, have you?
While on your way up the scale, if you sweep pick, you'll pick up/down/up, UP/down/up, UP/down/up, that is, you do two similar pick strokes consecutively while changing strings. That's all there is to sweep picking - nothing more nothing less.
When this method is approached correctly it's most logical way to pick. All of us have learnt in school the shortest distance between two points is a straight line. What we do in this method is to directly go to the string we want to pick without getting tied down by the down/up/down approach which is strictly followed in alternate picking method.
But some players tend to feel more comfortable while remaining in the predetermined pattern of alternate picking. They say it leaves them with more free time when they are improvising since the picking motion becomes almost automatic. I feel this to be more a matter of perception than any real problem.
There are also certain other common problems which many players face and generally tend to avoid sweep picking.
The most commonly heard refrain is sweep picking causes the sound to be sloppy. This happens not due to any inherent flaw in this technique but because most of them try to practice too fast. If instead they start off by picking slowly they'll get crystal clear sound. Speed is not a big issue. It can be easily generated once picking is perfect. The other real problem is inability to mute the notes that are not played. This obviously distorts the sound quality. But you can overcome this easily by pressing either your palm or thumb on the strings that are not picked.
You shouldn't sound as if you're strumming as your pick runs across the strings. Even though the arm moves in a continuous sweep, play the arpeggio slowly so that each note is roundly defined and clearly heard.