Sous vide effectiveness without using a vacuum sealer?
The main problem I've found is that air pockets can cause the bag to float, causing uneven heating. If your bag sinks, or if you have something to make it sink, it should be fine.

(Note: I find the water displacement method to be kind of a pain and not really effective. I use a cheapo manual vacuum pump, which I got for about ten bucks. It's very slightly more effective, and cheap.)