Personally, I love Vocaloids and think that they're a very creative programme for anyone to use. Whilst they don't sound exactly human at all, they're as close you can get to a human sounding voice without using a true singer or singing yourself. (That said, I do think that there's no replacing a real singer, even with a programme.)
As for favourite Vocaloids... I like SF-A2 Miki, Gumi Megpoid, and Piko Utatane the best, and think that they need a lot more love. It pisses me off a little when I see people comparing 'loids from other companies to the Big Six from Crypton (that is, Miku, Rin, Len, Luka, Meiko, and Kaito). Not every Vocaloid has to be compared to the Crypton 'loids, y'know.
And as for favourite composers, I'd say Hachi, DECO*27, and samfree are my three guys. In case you want to have a little taste of each of their music, here you go:
Hachi- "Matryoshka"
DECO*27- "Coward Montblanc"
samfree- "Luka Luka Night Fever"