I used to be a fan of clen before I use salbutamol which is very similar drug and should be available just like clen. Major difference is the shorter half life so it makes it easier to test higher dosage without risking of overdosing. It also has less side effects in my experience and results are pretty much identical, keep in mind I was never comp shredded, but more like legit 8% couple of times using it. So clen might have some slight advantages for people interested in getting crazy shredded. It is also available and prescribed for asthma (at least in my country) unlike clenbuterol which is banned and definitely not prescribed in any way. If I had to choose now, sal wins, if back then I'd say clen, it is the case of unless you try it for yourself.
I can't comment on clen long term, I usually used it for short term, since back then it was bro science method of cycling it like 3 on/3 off or something similar. The main thing however is like with most drugs is it depends on you as individual, how tolerant you're to stimulants or how you can handle strong stims. Just remember that dose up to the point might be better, but diminishing returns come fast. I'd advise to not cross 100mcg, not much benefit for more side effects.
Beware of higher dosage muscle cramps, I used to cramp up badly during the night or in the gym doing abs. I had to do some awkward poses because of cramps that were unbearable. The solution is to take 4-6 grams of taurine and balance your electrolytes while also being hydrated. Taurine is somewhat cardio protective too, since heart contains lots of taurine and clen depletes it, if I remember correctly.