It contains lots of old tunes that you can giggle about listening to years ago, laugh about, and remember with an odd sense of pride (or more likely embarrassment!) that you knew all the words to them. It boasts the Grease Megamix, (remember Grease!), the Spice Girls (cringe), Billie Piper (that's probably not a good thing!), All Saints, Aqua (thankfully not the Barbie Girl song), Catatonia, Eagle-Eye Cherry and Natalie Imbruglia. The last three are especially good to wail along to.
Overall, Now 40 is just another one of the Now classics, making it a required part to your CD collection. And, especially considering it's one of the better Now CD's.