I'm going to say this as nicely as possible but learn to use your shift key, especially at the start of a paragraph.
On to your questions though, there's no reason the LS block can't be reliable. It's a solid mix of proven design with modern technological improvements.
Manuals are geared different than autos, we don't have solid mpg numbers for any of them yet though so don't worry about whatever the difference might or might not be.
As for the engine, it's high displacement because it wasn't designed for little girls! Seriously though GM's LS block doesn't take any more space or weight than many 'smaller' engines. GM has a 2.0 liter that puts out 260+ horsepower, but it's a very different design. As for Ford their 4.6 liter puts out a piddly 300 horsepower while they use a supercharger on their 5.4 liter to get 540 horsepower. Throw a supercharger on the LS3 and you have the LS9 used in the ZR1 which is 638 horsepower. A full comparsion would take a long time, suffice to say my opinion is the LS block is more impressive, mainly since Ford's V8 is to heavy to even come close properly balancing a car, the GT500 for instance has a ridiculous 58/42 weight distribution problem. Some Ford fans love their heavy block anyway as it does have advantages.