the game is fun. i played it already twice. but i do agree that the loading times are really long that you are forced to watch the damn cutscene and that the slow menu is a little pain in the ass. and yes, the story really just ends and is unclear what was going on there. but hell, the game as such is really fun, just shooting things up is enjoyable.
after all you can discover secrets and you have tones of enemies coming towards you.
i also dint had any probs switching between the weapons. it was pretty easy for me at least and the upgrading works fine too. once you finish the game, you can play the entire game with the same upgrades which makes the game even more enjoyable.