I'm not a Sup Tech, but they do work for me.
A large part of any job is what you make of it. The role of the Sup Tech is to manage the receipt, issue, warehousing and disposal of all the stuff we need to support operations and can include spare parts, fuel, rations and stationary. There are many facets to the occupation and the skill sets are readily transportable to the civilian work force.
In the Army, in addition to the the supply stuff, Sup Techs will also be involved in defence of their position, combat logistics patrols and other activities that will require you to carry a weapon and perhaps even use it. In the Navy, you will participate in fighting the ship during action at sea and you can also volunteer to be a member of the Boarding Team (real door-kicker, Rambo stuff) or a diver. In the Air Force, they let you help choose the hotel ;D.
BUT your job, first and foremost will be to be a Sup Tech. This will mean many hours working in warehouses, counting stuff and dealing with somebody whining as to why they can't have an extra pair of socks. It can be very rewarding work (it's essential in order to achieve mission success) and there are many opportunities for advancement (it's one of the larger occupations), but it's not often viewed as one of the "cooler" trades.
Whatever you do, do NOT join as a Sup Tech if you're only seeing it as a stepping stone to getting in the CF and intend to immediately OT as soon as you can. The opportunity to OT may never come and you may end up being a Sup Tech for a very long time, or at least until you quit in frustration or get thrown out. If you're going to join as a Sup Tech, assume you will be doing it for a long time and be prepared to jump in and make the best of it. If you're not truly interested in being a Sup Tech, don't do it. You will only annoy those around you.