I always disliked the character of Andrew as I found it slightly homophobic in how everything about him was played for laughs. He was the guy who when the heroes were out fighting was hiding at home in his pinny and rubber gloves washing dishes. His obvious crushes on Warren/ Xander/ Spike were always used for cheap giggles. There was none of the respect that Willow/Tara got. They had character outside of their sexuality and their romantic life was always treated as seriously as any of the straight characters. So to be honest I find it slightly disturbing that a gay actor would so willingly contribute to such a stereotypical character.
(Not to say anything about the fact that the first chance the Angel writers got they turned him straight.)
Andrew was a weak character no matter his preferences. To me, he was JW's version of Gollum, albeit one who survived when he maybe should not have. Xander's last comment about Anya can be seen as being far more snide towards Andrew than her. His devotion to the heartless Warren was sad, whether in friendship or love, and he killed the only person who could stand to be around him (ala Smeagle's cousin). If TL had been directed to speak in a less stereotypical way (my brother had to point him out in Transformers) Andrew might have worked better. Everything in BTVS S7 was more than slightly overdone, in MO.
Andrew was the next generation of Xander, despite whatever his sexuality was. Xander had grown up and his humor was becoming more jaded due to his experances. Andrew replaced that, by bringing back whimsy to the show in a season that most needed it. Xander couldn't even cry for Anya upon her death, while Andrew did.
I think both Gojirob and Caleuche have interesting points here.
Andrew always has reminded me of a Gollum-style character; particularly with his arc in s7. In some ways, he's also an analogue of Spike and Willow in season seven (trying to atone for their sins), but presented in a more comical manner. His killing of Jonathan was particularly 'Smeagol and Deagol' like. The LOTR reference even gets a nod in Angel, when Andrew compares the resurrected Spike to Gandalf the White.
He is also in some respects a replacement for the early Xander of seasons 1-3. Whereas season 6 and 7 Xander is more mature due to his life experiences, Andrew gives us a slightly more whimsical character again.
See, I loved Buffy. All of it. Whether intentional or not, the story arcs eventually wove together very well indeed, so it felt like things were planned out all along. Season six is still controversial. There seems to be little middle ground. Some loved it for the darker edge and greater character focus; and some missed the lighter tone, and disliked the darkness and depression. I belong to the former camp. Season 6 was about the characters moreso than any of the other seasons in my opinion.
Season 7 returned some of the earlier humour and camp to the show (thanks mainly to Andrew). I loved s7, I still don't get why some hate it.