You can use a formula to figure out if it is worth it for you, that is what I did. Here is how I calculated it.
Let w = Tank cost
Let x = Tank VIP cost
Let y = Tank Hydro cost
Let z = Tank fill cost
5vips($x/vip) + 1hydros($y/hydro) + (15fills)(5yrs)($z/fill) + 1tank($w/tank) = Cost to own 1 tank over 5 years
vs.
(15tanks/yr)($15/tank)(5yrs) = $1125
cost to rent 1 tank 15 times for 5 years (given the price to rent doesn't change)
We tend to do 2 dives each time that my dive buddy and I go diving, so I bought two tanks, here is how it worked out for me.
2 tanks at $50 each (Used PST LP 72's w/ US Divers valves)
$75/yr to join a dive club where I get free air fills
$9/VIP
$20/VIP + Hydro
If my tanks last for 5 years here is how much they cost me to own. Given prices do not change. I also regularly get my tanks overfilled to usually around 2750psi after cooling down, the service pressure on them is 2250 and they are 65cf at 2250 so I get around 80cf at 2750.
2tanks($50/tank) + 8vips($9/vip) + 2hydros+vips($20/hydro+vip) + 5yrs($75/yr) = $560 over 5 years
Cost to rent 2 AL 80's at the cheapest I've found here without joining a dive club is $6/tank and I've already logged about 25 dives since getting qualified 6 months ago if I keep it up that makes about 50 dives/yr. So to calculate I'm going to multiply...
($6/tank)(50 tanks/yr)(5yrs) = $1500 over 5 years to rent
I get some additional bennies being in the dive club, discounts on equipment and classes (10% off), bi-weekly shore dives where I can rent two AL 80's for $1 each or I could just get my tanks filled and use them and discounted boat dives, I think $55 for a two tank boat dive.
Some things are assumed, like my tanks passing their hydro which is due Sep-2010. Their born on date is older than I am as first hydro is 1980. They're pretty old so I'm hoping they'll still hold up for another hydro next year. They've been used regularly as I bought them from a dive instructor and they have about 4 or 5 hydros done on them previously. The VIP is due sooner and given I get a discount if I do Hydro + VIP at the same time I probably will do the Hydro ahead of the due date when I do the next VIP. Even if my tanks fail I can still salvage the K-valves for use on another tank. New K-valves are about $50 each, you can get older ones like the US Divers ones cheaper though. I've also excluded any cost to have the valves serviced, I think K-valve service runs around $20 or so, I have a bunch of extra o-rings. Since they're steel I regularly wash them down after diving with fresh water and store them with about 300psi inside standing upright. The cost to have them tumbled if I get flash inside is like $40 or $50 I think but with good maintenance practice I'm hoping to avoid that.
Tanks well maintained can last a very long time. Mine are proof of that as they have about 29 years of use and still work great and are free of pits and corrosion. The longer you use the same tanks the more you'll save.