The short answer is that it depends on the airport, and on the day and time of day. Not all Class Bs are equal, and traffic density differs with the seasons and times of day.
On paper, SeaTac is fairly quiet for a class B; it sees about 945 operations a day on 3 runways (the third one being recent, opening in late 2008). The problems with the airport are in the ground facilities layout; every terminal that is the origin or destination of a flight at SeaTac is on the east side of the runway complex. That means up to 2/3 of flights require at least one runway crossing during taxi, and with a plane taking off or landing every four minutes on each runway, there's no time to dawdle. Very generous ILS holds due to the varying runway lengths further congest the usable taxiways. One GA pilot, unfamiliar with the traffic flow, can really throw a wrench in the works. As Jason Malinowski said in his answer, if you just want to touch and go they can probably get you in on the newest runway, but if you're planning on parking there you'll likely be staring up at the south end of a passenger jet for a while.
By contrast, an airport like Sky Harbor, which sees 1,200 operations a day on the same number of runways, is more efficiently laid out, with a center passenger terminal complex between the two longest runways, and most GA facilities located to the south of the complex along with cargo and ANG terminals, all conveniently next to 7R/25L (the shortest runway at 7800 feet, more or less reserved for small craft). Sky Harbor therefore tends to be a little friendlier to small singles and twins on XC flights (or just logging Bravo time for a student/rec endorsement).
DFW, larger still (1,800 ops/day), could easily be thought of as two different co-located airports with a common road system. Runway 17L/35R on the east side is primarily used by GA craft including small props, with the other four primary north-south runways serving large passenger flights (typically the inside runway on each side is for takeoffs and the outside for landings). However, the winds in the area are less constant; when a front from the Gulf brings winds out of the southeast, the entire airport runs on just two auxilliary 13/31 strips. In that situation, the East Tower all but closes to small craft, and nearby Love Field as well as the numerous Class Ds in the Metroplex (Addison, Dallas Exec, Spinks, Meacham, Alliance) see a significant uptick in small props and business jets.
O'Hare is just busy; no two ways around it. When the winds are favorable it has four runways on which to handle about 2400 flights a day (that's a plane taking off or landing from each runway every two and a half minutes on average), and when winds are out of one of the secondary compass points, each of the two usable runways will see a plane rolling down it every 75 seconds. A quick scan of the complex on Google Earth shows exactly one small twin in the entire complex, at Signature Flight Support toward the northeast. Everything else I see is at least a 50-place regional. They'll take you, as Fab's answer proves, but I'd be wary of duplicating his flight with the winds out of the southeast (he had the fortune to fly in with the winds from the east and thus the four primaries in use). Midway, closer to downtown, does seem to cater more to small props, but even then with Southwest's expanded schedule into this airport I imagine you'd be in for a fairly closely-spaced approach. As a result, I'd say if you were planning a private flight into the Chicago area, you should plan on landing at one of the class Ds, like Chicago Exec, Gary or Waukeegan.
