Plot/Story
Tsuna is a good-for-nothing boy. He's the worst student in class, can't play any sports to save his life and never talked to a girl. Typical shounen main character? You bet. That's, until his mother hires a 1 year old home tutor, who says he is the only heir (alive) of a mafia boss. UH?!
This manga is pure non-sense. There are mafia guys (and gals), hitman babies, diying will bullets, poisoning food, powers (and hairs) that defy physics-laws.... Pure cliche? Not at all.

Characters
The reason why I like this manga so very much. The characters. Oh they're not only funny, but also AMAZING. There're the most kick-ass females EVER (and I don't usually like girls in mangas). The mafia guys are WEIRD - you'll find guys trying to get attention of the new boss blowing up half of the city, guys thinking it's jsut a game, sexy girls in love with babies, babies with afros - yup. It's pure awesomeness.
And I didn't even start to talk about the bad guys yet...

Drawing Style
The drawing style is not THAT beautiful. It's kinda common. I was going to give a 3 out of 5, but then I remembered how the mangaka can draw COOL guys and gals. The poses are sexy, and she has a BIG sense of fashion. And yes - the characters CHANGE CLOTHES!!! That's a plus. Very original, heh.

Enjoyment
The manga is divided in some arcs. The firsts volumes are only the presentations of the characters and some gags in daily life. Then, there's Mukuro's arc, where they finally got an enemy, more adventures of daily life, and then the Varia's arc, where more of the Vongola family (aka the mafia) is involved. I find the daily life arc more enjoyable, because it's dorky and funny like hell - I guess the jokes work more for me than the serious part, even though the manga don't take itself seriously most of the time.
Most of people I know like the daily life less, but then you should read and see what scratches your fancy.

Overall
REBORN! is very... exquisite, at least. If you like classic shounen full of jokes (or if you are going to look for all the pretty characters), but wants something different from all the YuYu Hakushos and Dragon Balls, this manga is great and very amusing. Go for it.