[Subarunyon]

Others have said a big chunk of what I want to say (mainly the good parts) so I'll keep it brief while making it clear why the two last arcs aren't as satisfying to me =x

First / Second arc = 10/10 (I'll give it more if I can)
Third / Fourth arc = 7.5/10

The big letdown in third / fourth arc for me is the lack of consistency / creativity. What's missing from 3rd 4th that was in the first two arcs?:
Amazingly fun eyecatches - Arguably 3rd still has alot of good ones, but after the rossiu suicide eps it just went downhill. It's like they suddenly said: "Lets put the eps' main character's face on close up and put someone on the side!" riiiight.... From what I'm reading in the forums it seems to be fine for most of you, but I think this is just lazy.
Laughs - 3rd and 4th just isnt funny... I don't find the jokes all that entertaining anymore, and the dialogues seem to play less and less weight storywise =/ I dunno with you guys but I find Lord Genome / Leeron technobabbles very boring.
Pacing - I personally think slowing down after the GARfest that was 15 pretty annoying. When it does pick up steam again, the flame just isnt that bright anymore. Maybe the mistake is 3rd arc after all? I think if they skip right to 4th I wouldn't have any problem with this. Or say, maybe give a one season break before 3rd arc.
Meaningful deaths - The only good one in 3/4 was Lord Genome imo.... Kittan lacks development, and the rest of the DGD felt meaningless. Nia felt like an LOL GAINAX more than anything.

Bottomline: Third/Fourth all in all just isnt Gurren Lagann enough for me, not including the two Ark-GL eps and the two final eps.

Characters:
Simon - never liked him much... I find him a bit too... generic? It's not that he lacks development or anything, he's just not so interesting / intriguing to me since I felt like I've seen him in some other anime before alot of times =x I think the way he was handled was really good tho
Kamina - Definitely my fav char in the series, I don't think I need to say much. He's a breath of fresh air in a world where most of the main characters are wimps that thinks way too much.
Lord Genome - My #1 fave in terms of development. The way he returned from death was surprising to me (one of the few original things in arc 3/4) and the way his story comes full circle in the final eps was the best in the series imo. I think his story has the best closure in all the animes I've watched. He failed as the defender of his tribe and have to become the opressor, then he was given a second chance until finally he regains full body and even used his asskicking mecha again... but the best is saved for the final episode; now THAT's GAR. I think his final interactions with Nia was really good too. I actually find him not saying anything to Simon about Nia really interesting; him sacrificing himself to become Simon's Drill really felt like acknowledging Simon as his worthy son-in-law without actually saying it. I wonder why no one talked about him at all in the final episode thread, I guess we're all too busy crying over Nia =(
Rossiu - For all the rants I said earlier on 3rd and 4th arc, Rossiu is strangely the character that I can identify with the most. I'll say it now: I actually really like the 3rd arc, just not as Gurren Lagann. If it'd stood alone as an ova / a separate anime, I would probably give arc 3 a 9/10, but not when it's in GL.
Gurren-Lagann - The way the mechas were introduced (First gattai) and the techniques that it uses was just so fresh that I just kept coming back for more. Too bad the freshness only lasts up to 15, (+ vs moon parts) after that I felt like I've seen everything =/ Chouginga and Tengen Toppa just doesnt do it for me. It guess have to do with the pacing too.... Nevertheless, Gurren-Lagann will stay as my top 5 fave mecha of all time.
Yoko - Don't really like her other than in the Yomako sensei ep. I was surprised that so many ppl actually don't like that eps. I found her a lot more interesting and much less cookie cutter there =x
Nia - Nia is my fave char after Kamina. I guess this is another reason I dont really like rd / 4th, Nia feels really underdeveloped there when her journey has just started. and heck, WHY do they have to kill her? The only valid reason I've heard so far is that if Simon chose to revive her, that would be like letting spiral energy going out of control and destroy the universe, but they didnt even get that across very well. Even then, I think there should be better ways to say that without actually killing her

Well, all in all, I give the series a 9. It's not perfect, but definitely one of my top 10, and one of the few animes that made me want to get the DVD =)