Recently, I purchased a new table saw. It's a contractor style, so the motor hangs out the back, which is gaping open. I have just moved my new saw into the basement, it used to be out in the garage, so good dust collection is a priority.
If you hook up a dust collector to this style of saw, then you're going to have to do some work to close up the back of the cabinet, or the suction will, if you'll pardon me, suck.
I've read about a few different options for closing up the back of the cabinet. Dave Wooland at The Saw Shop sells a fabric "boot" which closes up the back of the saw. I've heard read some good reports on that product.
Also, I've read a magazine article, which I think was in Fine Woodworking, about building a cover for the back of the saw. It's a complicated custom-built jig, with curved slots in it to allow for the motor to tilt.
I took this last option, and simplified it a lot. I rarely use my saw for bevel cuts, so I just built two wooden covers which fit around the motor mount and do a 95% (or better) job of closing up the hole. They are held on by friction and rare-earth magnets. If I decide I need to do some bevel cuts, it's a simple matter to remove them and make the cut.
A zero clearance insert will be the next step, to help keep the sawdust in the bottom of the cabinet. The final step would be to build or buy a blade guard with an integrated dust collection port.