deca sticks around in your system a long time which delays recovery. until the body knows its out it won't start the recovery process. there isn't any AAS out there that sticks around as long. i would say a good 5 weeks after last pin you will still get results from deca. thats why people say run test longer, i would even suggest running test 3 weeks longer. since deca is detectable in the body 12-16 months after I would even argue that it sticks around in some form longer than 5 weeks in very small amounts.. enough to mess with your HPTA (although I have nothing except experience to back up this theory) half life is 2 weeks but that doesn't mean it doesn't stay that long.. anyone who has taken an oral for example knows most have half lives that are in the hours, but even after stopping taking them you keep getting results days later. so don't be fooled by thinking half life is how long it works.
if you read the link in my sig i tell you what to run with deca during cycle and post cycle to recover.
as far as sides they are in fact LOWER than other AAS, but it depends on what you mean by sides. will it de-hydrate you like some orals and tren? nope. will it give you insomnia ? nope as long as you run an AI with it. there are variables. deca is the gyno king so if you are prone better stay away. a lot of these young guys at the gym with bitch tits and wide shoulders you can tell ran deca with no ancillaries out of ignorance.
IMO deca is so popular cause the old timers loved the fact that it stuck around so long in the body after the last pin but I would NEVER run deca again. after a cycle I don't want to spend 3 months trying to jumpstart my HPTA, no thanks. been there, done that.
if you want to go that route atleast go with NPP which has a shorter ester, just like i always recommend guys run tren ace instead of E. also the name DECA sounds hardcore almost like you are DECKING someone so it attracts the meatheads.
to answer your question on what makes deca so great. i would say the slow and permanent gains that you get. and the crazy appetite it gives you. since food is the most anabolic thing in existance you get the best of both worlds with deca.. its a bulker pure and simple. you will be eating like a pregnant women.
run it 8 weeks max and run test either 2-3 weeks further.
300mg with 500mg test. i would also suggest kickstarting with dbol (you only need 20mg), and finishing off with beastdrol 20mg, 30mg if you can handle it (which will take your strength to another stratosphere towards the end of the cycle)
run cabaser while on.. .25-.50 every 4 days.
run forma while on and into PCT. follow directions on label to combat estrogen and progesterone
run hcg during cycle.. 250iu's 2X per week. then up it to 1000 iu's 2X per week after last injection until the test clears your system. run 2 injections of HMG along with the hcg towards the end
then start SERM therapy 12-25mg ED clomid for example.
run unleashed, post cycle with the clomid to offset any sides.