Yes, Marks is extremely reliable!! I've gotten my Lantus from them 3 times and probably hundreds of other members here (and on other boards) have too since I originally "found" them several years ago and shared the news!
They have great customer service too. It takes about a week for shipment once they get your script and information, and they'll give you a tracking number that you can watch as it travels through the postal system too.
It's packaged very well (with a medical grade ice pack) and has always lasted to the last drop. (there's a link to a video in the original link above that shows how they ship)
Most of us use the Lantus Solostar pens because the vials 1. break so easily and 2. usually the insulin loses efficacy at about 4-6 months, even if cared for properly. The vast majority of cats here are on less than 2 units twice a day, so a vial would have to last 250 days (longer if your cat is on even less than that) or you'll end up throwing a lot of it away
The pens are just like "mini-vials" that hold 3ml (300 units) instead of 10ml (1000 units) and a package of 5 will give you 1500 units (enough for about 2 years for most cats).
The pens don't expire until the date on the box...which is usually at least 2 years away, so most of us can use every drop.
We still use regular insulin syringes with the pens, not the special needles that humans use so we can get doses other than whole units. When you pull the cap off the pen, there's a little rubber stopper just like a vial. You just use the same insulin syringes and pull out the correct dose.