Hi...my friend and I spent 2 nights in a JJB in Seoul (no idea which one, it was our first time in Seoul, we just bumped into one), and yes, you can store luggage in some of them (ours you could...it was one of the reasons we were happy with the place). My friend had a massive 70L backpack (he was on a 1-month trip) and I had my little 30L backpack (I live in Korea, was just doing a long weekend up there). Our lockers were big enough to fit our bags without issues.
The good:
- Yes on the lockers
- Cheap (we paid 10k won - about $10 - per night per person)
- Since we paid up for a couple of nights in advance, they let us store our luggage there during the day, which was a nice bonus.
The bad:
- It was slightly hotter in the JJB sleeping area than it is on Venus, but with 970% humidity.
- There were about 150 people in one open area. If anyone snored, talked on their phone, etc...you heard it.
- Sleep on the floor with no bedding of any kind. We got one brick of a "pillow" - it was a block of hard foam about the size of a dictionary wrapped in coarse vinyl - per person and that's it.
It really depends on how long you're going for. I've been back to Seoul plenty of times, and found love motels/yeokwons (they're surprisingly nice!) for 30-35 bucks a night. For a weekend, especially split between people, it's a no-brainer over a JJB.
Edited by: Canonite