Mass Effect 2 has a lot of characters. This is part three.
Tali'Zorah Vas Normandy
Tali is the second returning characters and, like the first, she is far more compelling than many of the new inductees. Although you meet up with tali on the first stage, you don't get her to join until later. Thankfully when you do she has a very compelling mission. Then she has an even more enjoyable loyalty mission. Basically she is one of your most reliable, supportive, and intelligent crew members. She is also a romance option, however, I play female and she's not interested. For me, that is best because I think it is better to just stay friends. When it comes to combat, she's fantastic. Her shield drain ability is perticularly fun. In my opinion she is the best companion in the game.
The problem with Legion is that you do not get him until the very last minute in the game. This feels to me like a last minute design change and I do not think it benifits the player at all. Since you don't spend much time with him, and the controversial nature of the charater, it is difficult to trust him on the suicide mission. Since that is really the only thing you can do with him, that doesn't leave much reason to have him around. Personally, I enjoy the insists he give you into the geth and see potential. The main problem I have is the conflict he will inevitably have with Tali. It is very difficult to get them to drop the fight (which it should be, I wouldn't expect them to), but the only option to retain their loyalty as a renegade character like me is to lie to Tali... I'm just not comfortable doing that. I usually turn him over to Cerberus for the money.
Just a quick note, DLC characters only have a loyalty mission and have a different way dialog works. They're largely unnessecary, but they can be enjoyable.
I really like Zaeed. It helps that he was free for people who bought the game new. Since all he has is the loyalty mission, it helps that his is awesome. Full disclosure though, I've only played it renegade. My favorite Zaeed moment is watching him light his former partner on fire in his loyalty mission. He's such a bada**. I would not want to miss that. If you take the time to talk to him, he'll share a lot of stories of his days as a merc. He has a great voice actor as well. He seems like he is not a nice guy at all, but maybe you can convince him he still has some value as your personal soldier. His combat skills are great! Disruptor Ammo pairs well with his Grenades. I like to give him area grenades. It is the best ability for taking down large groups IMO.
Kasumi is a nice, but not really needed addition. She came later so I think most people missed seeing her. She's a thief and her loyalty mission is very different than anything else in the game. You cooperatively break into a rich criminals private museum. In addition to the coolness of the mission you get some really powerful SMGs to use. At the end, her preference is to obsess over her old boyfriend. I understand they had something, but now that she has his data, I think it is time to move on. In combat she has an ability that allows her to stealth up to enemies and punch them. Its very strong and makes her worth taking along. Sadly, I think what I enjoy about her most though is the addition of a personal bar in the Normandy. I don't think that says much for her character.
So, there it is... my professional opinion on all of ME2 team-members. I think it is too many and am glad they're reducing it. I hope ME3 has more interaction between characters and more development for characters that your could romance but choose not to.