Wanted to chime in on this one as I have a good bit of experience with Sous Vide and have some things that I've discovered over the last couple of years doing it to add to the conversation.
This is critical. It doesn't matter if it's fillet or the toughest piece of beef known to exist ... you need proper fat component for things to work. The leaner meats, like fillet for example, need some help. For these types of sous vide cooks I keep around some rendered beef fat -- I always render up my own when trimming up large pieces of beef (I often buy large wholesale pieces cryovac style from Costco or the butcher and do some of the end bits myself). Rendered beef fat for beef, bacon grease for pork (and even some beef too, depending on the flavour you are looking for), and duck fat for any poultry ... these have been the best combinations in my experience. The "tougher" bits of meat tend to come with a good bit of their own fat in the marbling ... but it doesn't generally hurt to add more to these ones either.
Butter, while yummy, is actually not a great choice for any long/slow cooks, nor is anything else dairy. Even though you're not passing the temperatures required it often breaks. If you want to add this type of component to your bags before cooking use some evoo. Even for fast cooks I try to avoid butter. Try doing some A/B if you are skeptical of this and you'll find that the butter really changes things in a bad way.
I feel very strongly on this one as do a lot of high end chefs apparently. DO NOT pre-salt meat before cooking sous vide, at least not directly. If you have meat that you feel really needs the salt ahead of time brine it, but again DO NOT add salt directly to the bags. This massively changes the texture of pretty much all meat when using this technique, and generally not in a good way. Again if you are skeptical on this one do some A/B testing for yourself. And yes I know that things like beef really really need to salt ... but you would be surprised how far some really nice finish salt goes after you're rested and sliced. If you really feel the need to get it in and through the meat before then you can inject at the resting phase as well (in fact that might be a perfectly appropriate time for butter depending on your tastes).
Again this one is a no-no. If you need to use garlic for sous vide use powdered garlic. Fresh garlic will turn very metallic and will change the whole flavour (and sometimes even the texture) of your dish, this is amplified by longer cooking times.
Long and Slow vs Fast Cook and Temperature choices
This can be a doozy and I won't try to cover tons of it here, but a good rule to follow is to base your cooking time on a) how much fat content is within the meat itself (not added to the bag) and b) what kind of texture are you looking for? I see lots of people online that just assume if you cook anything for a ton of time sous vide it's going to make it better. That can't be farther from the truth! Lean cuts will cook quickly and will not benefit from longer cooking times -- in fact they will quickly get mushy and you will ruin the flavour of them. Things like fillet/tenderloin, prime rib, strip loin, and even round only really need to come up to temperature. Save the longer cooks for tougher bits that have a good bit of collagens to break down (like brisket, blade, etc). Cooking a lean piece of meat for longer than it takes to come up to temperature is going to do nothing except ruin it's texture, but cooking a tougher piece of meat for an appropriate amount of time on a long cook and breaking down all the collagens can do wonders :)
And as probably has been mentioned many times all over this forum ... FOOD SAFETY IS VERY IMPORTANT! Especially with anything you are doing long and slow. Keep a very clean environment when working with this sort of stuff, hit your target temperatures within your safety window (get up to 140 within 4 hours max!), and when doing large pieces of meat sous vide there are really easy to use and cheap to get apps for your portable devices that will help with time calculation (and even show you graphs on when all the pathogens are dead, etc).