They only leak out the sides if you pour your water too quickly and too much.
I use a garden sprayer on my autos (my photos are auto watered and I don't do anything to them in regards to watering anymore). What I do is mix up 2 gallons in my sprayer, then typically for me, I have 2 plants that need watering at the same time, so I'll start watering one of them, turning the plant as I water. When the top layer is nice and saturated, I'll switch to the next plant, and then repeat until the grow bags are nice and heavy. I get very little run off, and takes about 10-15 mins to water the pair (they're in 3 gal fabric pots, 5 gals would take a little longer).
I prefer fabric grow bags because it's easier to tell if you're being aloof while watering, whereas plastic pots you won't be able to tell so easily. That's just my preference, though, and there's no wrong container to grow in.