The second you start to uncloak the enemy can indeed see you.. they can see you even when you are cloaked, they just can't get lock on. Without lock on you can't fire missiles or plasma at a target, but you can fire direct fire weapons i.e. phasers, at a huge range penalty.
Cloaking, at least in SFC, isn't all it should be. Most Romulans I know almost never cloak. They just use the extra power the ships have for cloaking in other ways. If you have to cloak.. something you can do is try to stay behind the enemy and uncloak behind him (less weapons pointing in your direction) be ready to recloak if does an High Energy Turn to face you. Then just work your way back behind him again. You can cloak and uncloak a whole lot more than he can HET.
SFC is a great game... One thing I think that did hurt it was a steep learning curve. Once you start to get the hang of it, it gets a lot easier.. until you face a seasoned HUMAN pilot. The best computer ship is NOTHING vs a human.