My Initial Reaction...
Man do I wish I had read The Assassin and the Underworld before reading Throne of Glass! I have a feeling I wouldn't have seen some of those twists coming. And I would be happier right now about some of the developments.
The Good...
Is EVERYTHING a good enough answer?! Okay, yeah that's not entirely true either, but Gah! This was good! I'm such a character driven reader and the character development in just 100 some pages is just incredible. We've been getting to know Celaena in the other ToG novellas, but in The Assassin and the Underworld I feel like we're getting a feel for the other players better than ever before.
After reading ToG I thought I was all about Chaol and Prince Dorian, but now I get why she's not falling all over them. She had someone even better! Sam Cortland people! Okay to be fair, I haven't read the rest of the series and I may end up falling head over heels for one of those guys, but I official love Sam. He's strong and capable and wants to protect Celaena. But he doesn't think that protecting her means squelching her independence. He lets her be strong and capable all on her own. He just wants to be her partner. Is it possible that my feelings for him are a bit altered by having read ToG and knowing where all this is headed? Hmmm... possibly!
The Bad...
So only one thing really bothered me in The Assassin and the Underworld and it was Celaena's attitude toward Sam. She's so mean to him and sometimes I got it - she's hurt and on the defensive for a multitude of very good reasons. But other times I just wanted to reach in and smack her around (except that she'd obviously kick my ass, if not kill me LOL). She's supposed to be smart - this bad ass assassin, but she's not a very good judge of character - at least not that I've seen so far. Poor, shockingly naive for an assassin Celaena. This is kind of a good point too, though. What a complex character - assassin and yet still young and naive!
Concluding Sentiments...
Obviously I loved this, but some elements were very predictable. I think this is because I already read Throne of Glass, though. So, my recommendation: If you're new to this series, start with the novellas. I think you'll appreciate them and the book more if you read them in the right order.