I slotted several months with a dremel, happy with the results although a non-electric method was something I wanted to get to.
I tried several saddle valves, including nominally the good ones from Amazon, very unhappy with the fiddle factor (I just want to punch two holes, quickly, consistently, not damage the carto).
I came up with:
http://www.e-cigarette-forum.com/forum/pay-forward-pif/272989-diy-carto-punch.html Very happy with the results, longevity, etc. At post 69 you'll see a link to someone's thread on a different approach, also very solid, just drilling a small block of wood, a roughly 5/16" hole (for the carto) with an intersecting, centered, hole for a deck screw (for the punch screw). My metal hub likely has better pure longevity (the wood will strip over time from multiple insertions/removals of the deck screw), but either of these methods IMO are simpler, more consistent, faster, than a saddle valve. The wood block is dirt cheap, if you have the right screws and drill bits, my hub approach runs about the cost of two of the cheapest saddle valves (or one of the higher priced ones, but without the hassle of setting up the saddle valve, sharpening,...).
Punching cartos yourself lets you use whatever carto you want, within reason, and really is fast. I have complete consistency in results, a biggie for me.