I think its fine solo. Still a great game. Yeah certain bosses are a pain in the arse and the swamps of Corsus were no fun at all solo, but overall I actually found the game easier solo, but still enjoyable. In my experience co-op made the game harder and more interesting due to the increase in enemy health and numbers, and elites being more common. Co-op is definitely more fun than solo no doubt.
Also those big scary machete guys who block your shots? Not too hard if you have a shotgun or couch gun. When they go to attack, blast their face once and dodge away instantly, possibly will have time for a second shot. If they stretch out their arms for their pincer attack try to use the backstep dodge to avoid it, it'll get you more time to retaliate. They're actually quite squishy. As for Gorefist...well he just sucks, not a lot I can say about him :\
"Oh god not her!" [ Warning: Do not feed the chibi ]