I stayed here while in transit through London and had a great time. My intent was to use it as a base to explore the city and to have a bed to sleep in at night.
In terms of security, They have a luggage storage area where you can leave your bag in case you arrive early, this is a shared area but they also have lockers in the storage room which you can use for a fee. The rooms themselves have cabinets that can easily fit a 21" carry-on bag (with lots of room to spare, the cabinets have have a place you can put a padlock to keep your bag secure. The room area also requires keycard access, I had to use my card once to get to the room area, and then again on my room door.
The staff at the front also gave me a free SIM card, and map of the city. Two things that were invaluable during my trip.
The hostel is in a good location, a 5-7 minute walk to Oxford Circus. This was an excellent base for me to explore the major tourist areas in London.