are Laleli and Taksim good districts of Istanbul (in all senses)?

 Laleli is not good but Taksim is safe..It is my idea..

what is wrong with Laleli?

It used to be all sorts of things going there mainly  drugs, prostitiution..

I am not sure it is till the case but Taksim is always a  better place than Laleli..

 And how would you know this thehandsom?

them what is the best district of Istanbul?

I agree that Laleli is not really good unless you´re interested in buying clothes in bulk. Taksim is great if you enjoy nightlife and crowds and Sultanahmet is great if you enjoy historical sites and...crowds

thank you a lot!

what is best one to feel spirit of the city? besides historical centre?

I do not know if they are the most representative of the spirit of the city but my favorite are Eyüp and Fener ... especially in Fener district I like to walk around just to see the old houses and get in touch with people who is really friendly. For nightlife I prefer Kumkapý to Taksim ... much more characteristic.

 Those are the good choices for history oriented sightseeing... the old city, balat, fener are very good places to see... also across the golden horn, galata would be added and can be made a nice triangle...

for nightlife, i agree that kumkapý would be well for remarkable raki nights.. but you need a company for that... a group of 3-4 people at least...

but for more like disco, bars and pubs taksim is the best region...