My pre-op diet is different, so I can't comment on what brand to use, but you could buy a store one and take it to your dietitian for approval. The important thing is to follow instructions exactly. Not only will you drop a tidy sum of weight, but the glycogen stores in your liver will get used up, making it smaller, so your surgery will be much easier and safer.
I can tell you some ways to help get them down. I have a Keurig coffee maker and have found that dumping my shake into a big mug and zapping it for a minute, then making a 6oz cup of coffee into the mug (or earl grey tea with a vanilla shake - yum) gives me a yummy latte instead of yet another boring shake.
You can also make your shake into a frappe by putting it in the blender (Magic Bullet is great for this) with ice and some instant coffee or a sugar free syrup. (you can buy them at Starbucks, Second Cup and the Grocery People)
It sounds like you should be grateful for the change from Optifast. Scuttlebutt is that Optifast is vile. I haven't found a lot of difference in taste between the other brands.
My challenge is giving up caffeine, which I'll have to do until 3 months after surgery. I only drink 2 cups of regular a day now, but I'll miss them. I really can't tell the difference in taste with plain coffee, but all of the fancy flavours only come in regular. Darn.
Highest 303.4, Surgery 263, Current 192, Doctor's goal 180, My goal 165