Westjet has been in business for quite a few years and have been very reliable (as much as any airline can these days). They're the second largest carrier in Canada next to Air Canada. For many years they operated only 737's on domestic routes in order to keep parts inventory and training consistent to one type of aircraft. However in recent years they've expanded somewhat by offering service on shorter domestic routes using Q-400's (under a separate brand, Westjet Encore). They've also recently (this year as I recall) purchased a number of 767-300's so they could compete on the international routes. I believe they currently fly to Dublin and Glasgow, but will add Gatwick next spring. In the winter they'll probably switch their long haul flights to service the sun destinations (Dominican Republic, Costa Rica, Mexico, Cuba and Hawaii).
When they initially started service to Europe, they were flying Newfoundland to Dublin direct with 737's. I attended a travel event this spring and asked one of their Pilots about the long time over water in a 737, and his answer was "we're over water longer on the Hawaii runs so it's not a problem". Good, logical answer.
I haven't seen the specifics of their pricing model on international routes yet, but it could be similar to the "no frills" service of Air Canada Rouge, where every amenity is at extra cost (entertainment via iPads, no meals, etc.). I suspect the low prices that you're seeing now are a "loss leader" to promote their new international service, and they will eventually increase.
During my trip planning every year, I always look at a variety of carriers which includes Air Canada, Westjet and Air Transat, and I wouldn't have a problem travelling with any of them. However, I just about always find that Air Canada is the only one that has the flights and code share connections to get me where I want to go, at the time I want to go (that was again true this year). I did travel with a Westjet / KLM codeshare one year, and everything went well. This year my trip "over the pond" was Vancouver to London on Air Canada Premium Economy and they were among the most comfortable flights I've ever had (but not cheap).
In the same situation, I'd probably try the Westjet flights if saving money was high on the priority list, but at the same time realizing that they may have a few "bugs" to work out in getting their international service up and running, so there could be a few minor problems.