I have about 140 playing hours so far and barely 10 minutes cloaked.
If going Recon, I definitely prefer to have RDX and APMs equipped.
The Lambert is awesome, seriously, but I personally prefer a fighting approach over a sneaky approach.
The bad thing about the cloaking device is that you need to release the button and take out the RDX, knife or firearm which costs additional time in which the enemy might have killed you already. Also, as mentioned by others several times, you're quite visible when running. I say in 85% of all cases, the cloaking doesn't help that much. The field of distortion it creates is quite visible, even in a fight zone where people are likely to be distracted by other enemies.
You can't expect to run into an enemy, be it on foot or in a vehicle, full frontal and be successful. It is not that good. As said, you have a field of distortion and also quite often the netbat red rhomb.
The device is made for sneaky gameplay. Personally, I keep wondering if its necessary at all then, because if you need to sneak up on vehicles from behind or the side, you might as well do that uncloaked and have your RDX in hand already. Also, if you use it to cross larger open grounds, it is more likely you're not dying because the enemy can't hit you than because he can't see you.
Just IMO of couse.
Proud member of a dead community.