Thanks guys for Your advice, but maybe to make things clear I will describe the use of Suggestion I had the most problems with. My player wanted to know, whether the NPC was really who he claimed to be, and made a suggestion "Tell me please, who sent You" (as far as I understand this is a proper use of the spell, as it suggest the action not directly harmful to the NPC). NPC succeeded on a saving throw however and this opened a can full of worms. Does the PC know, that his spell failed? If not, does the NPC know he was being influenced and lie, tell the truth nonetheless or act in the other manner (eg. "Why are You so suspicious of me? I have doubts considering Your good intentions"). If the PC does not know that the spell failed, can he suspect a lie from the NPC? Then I also glossed over the description of verbal components and it seemed to me, that the effect of the spell (in this case the suggestion itself) is separate from incantation. That's why Sorcerer can cast spells without it (silent spell metamagic). Plus Suggestion already has material component built in (but I suppose You can rule, that grabing the component in Your pouch or under the cape is sufficient and won't raise any suspicions).
So, having this questions in mind, I ruled back than, that we would stick to the "Jedi mind trick" image of the spell that we had in our heads (PC knows if the spell fails, NPC doesn't know he was being influenced, suggestion is the verbal component). But is this really how the spell was designed? And is loosing spell slots really sufficient penalty for this spell failure?