I'm just not sure what you're all seeing wrong with the infiltrator. I've had plenty of success and I'd hardly call myself an expert... No, you're not going to be mowing down lines of enemies like a max, or carpet bombing like a pilot, or generally blowing up tanks like a HA, but if you're patient and take your time you can sneak into an enemy base and disable the generators just fine. Go find a vehicle station or something similar and wait till people walk up to use it then you sneak up and give them a couple of stabbity stabs with your knife (or empty a pistol clip into them if you're TR). Unless you're NC, the sniper rifle is slightly useless except to just harass people, especially if it's a big battle and they're too busy to figure out where any specific shot is coming from. It's good to look ahead though. Having a scope is always good for scouting. And I will say, even if you don't get any experience for it itself, a VERY useful, if underappreciated use of a infiltrator is as a recon, painting targets with the Q key and ideally being in a squad or platoon with various support assets. An infiltrator in a good, moderately hidden position with plenty of visibility can LEAD a fight if they just keep scanning the area and call out what they see.
Never move without your cloak if you can avoid it. If you're still, you're almost as good as invisible in most cases. Find cover and things that will break up your silhouette. Don't just pause in the middle of a bunch of white sand... get to a rock or tree or something and wait next to it. Hide in bushes. Try to plan your route so that you're cloaked most or all of the way between cover points. I went deep behind enemy lines one night and spent a good half hour just harassing people who were landing at a tower with air pads and repairing their planes. Only one guy actually found out where I was, but I spotted him a good half km out and just moved position. Could have killed him if the terrain had been right and I'd gotten slightly lucky. (Also, making him run out to search for me, and then have to run back was a bigger waste of his time than me killing him would have been just to send him back to the tower spawn in 5 seconds, but I wasn't thinking that hard about it at the time... just something to keep in mind.) I was only actually killed when I got greedy and decided to try to take the 3 points of that base by myself... Got to the second one and was cornered by an HA.
You're not going to be making xp hand over fist like other classes... but if you play it right, you'll die a lot less as well and be somewhat useful too.
Patience is the key... if you don't have that, then don't play infiltrator...