BBL worth it? Will the fat just shrink o go away completely after a couple of years?

There are a lot of variables that affect fat retention after grafting.  The technique of harvest and purification can affect the viability of the fat.  You can purify and concentrate the harvested fat to remove non fat liquid components using a variety of methods.  1200 cc of harvested fat might be 50% tumescent fluid and injecting this results in only 600 cc of actual fat put in (without processing to remove the fluid prior to injection).  Experts still disagree on the necessity of concentrating the fat prior to injection and on the actual method of doing this (filter systems, centrifuges, etc.)
There is a limit to how much can be injected in an area and expect to survive.  This makes depressed areas like hip dips challenging because the exact region you want to inflate the most has the least tissue to inject into.  If you over inject an area, the fat likely does not survive very well if at all.
There is not a lot of consensus regarding garments, length of time to use them, pressure avoidance on the grafted areas, etc.  and to my knowledge there are no studies that specifically look at this.
In my experience (and I generally place 250-500 cc of concentrated fat per side) the survival is about 50-70% in the subcutaneous plane at one year.  It is not uncommon to have to treat the hip dip area a second time.  I think I get good results with more modest grafting combined with precision liposuction of the hip and back as well as sometimes the lateral thigh area to create a nice curve from trunk to legs.  
I have revised the result on patients who went elsewhere for massive fat injections and ended up with suboptimal contour.  I think those seeking high volume fat graft to the buttock really need to do their research and find a board certified PS with substantial experience in this variation of BBL and a solid track record.  Also (and perhaps because I have been around a long time now) I caution those who seek significant exaggeration of their figure at an early age to be mindful that this can have adverse consequences to their appearance with aging.  

Fat is a great filler because it has long lasting results.  After a BBL, some fat will be lost during the first few months but after that it should last long term.  However the size of the buttocks will fluctuate with weight gain and weight loss. However it should not go away completely after a few years.

Modern techniques of fat transfer offer long lasting and excellent result.  For safety, BBL fat should always be placed in the subcutaneous/gluteal fat above the muscle.  Fat should be placed under low pressure and not overfilled which can result necrosis.