My uncle recently flew on them, and about 2 months ago my sister did. Basically they do fly with more seats on the aircraft for better seat-mile economics, but then again, AC/CP seem to give pretty good legroom when you look at the number of seats on their 2-class aircraft compared to US carriers.
Full fare prices are much lower on WJ than AC/CP, but if you plan ahead you should be able to do a lot better than full fare prices, so it depends on how you compare WJ with AC/CP.
WJ routes are flown less frequently in general, but it's with larger aircraft. Also, WJ doesn't especially go after business travellers.
In flight service is pretty basic. Drinks and a light snack, but it's really pretty minimal.
One thing about them my sister said was that although they try to make it a bit more of a lighthearted atmosphere, it's kind of irritating when they have the same tacky jokes on both flights...
One thing to remember is that WJ carves out some intersting niche markets. Abbotsford, for example, was opened up by WJ (although C3 has started into there now). Hamilton is a fairly good sized market as well, but it's also within the shadow of another airport, but WJ offers passengers the ability to use convenient alternate airports, not just the big hubs at YYZ or YVR.
Overall WJ is a very well managed airline, with good growth prospects, a good on-time record, good prices, and very well focused on it's no-frills approach.