I just managed to finally get the last companion and the change in gameplay was dramatic. I completed the story arc, which gave me access to my final companion three levels under the recommended level. But once I had access to her it made leveling and the other side quests a lot easier. I went from cruising around with my healer companion, slowly killing one mob at a time in a tanking stance, to being able to slice through four mobs in seconds. Having two Jedi DPS (myself and my Jedi companion) is amazing. I refer to it as lawn mower mode, the way the lightsabers are spinning, you just plow through anything that gets in your way.
The Shadow spec that I picked up has a stealth mode that's helped me complete quest lines much faster than I should have. Sneaking around can complete quests faster but it seems to result in less Experience and Loot, as you're sneaking past mobs rather than killing them.
I have found space combat to be a good way to assist in leveling up. You get a variety of daily quests you can do and I've found them fairly easy, up to about level 40. Ship upgrades are another major expense if you want any chance of being able to complete the level 40 combat missions, you're going to have to sink about 100 000 credits into upgrading your vessel.
The biggest issue I am finding at this stage is credits. I'm just scrapping by level by level with my skill upgrades. I have nowhere near enough credits for my next speeder training and at some levels I found I didn't even have enough credits to get the level up skills required. Using the standard speeder you get at level 25 is not that bad, but having the upgrade training blinking at you every time you walk past a Jedi Consular trainer, and not having the credits to get it, is frustrating. This is the same for my crafting and other bonus items, all available everywhere but out of my budget. I have to save all my credits just to be able to upgrade my combat skills. I am concerned that if the costs stay as they are, the credit farmers are going to have a field day.