An Obligatory Disclaimer:
I'll begin with an overly lengthy disclaimer: this show was NOT intended for those that are easily offended or upset by stereotyping, historical inaccuracies, or the trivialization of major historic events (also, homophobics beware: this show is laden with innuendo). Now, let me just say, to all the people that fall into these aforementioned categories but purchased the show anyway and now are complaining vehemently about it via review, this show is exactly what it makes itself out to be: a CARTOON. If you were looking for something serious, then good god what were you doing browsing through animated TV shows? Would you go to Spongebob for life lessons? No. Likewise, if you're looking for an accurate portrayal of historic events, go buy a historical reenactment. It's not rocket science.
That said, I'm going to attempt to answer some basic questions you might have as someone that is considering purchasing Hetalia: Axis Powers.
1.) I watched the Japanese version with English subs. What's new?
Aside from the fact that you can now listen to the English dub, the dialogue seems to have been edited rather severely while still maintaining the basic personalities of the characters. The English voice actors have also attempted to take on the accents of their respective countries, some more successfully than others. Happily, this version contains both language tracks, so if you get fed up with the English dub you can always switch back to the Japanese track.
2.) I'm new to Hetalia. It sounds interesting, but how do I know if it's for me?
Short answer? Google it. Just type in something along the lines of, "hetalia english dub episode 1." Watch the first episode, posted by Funimation, and you should get the gist of the show fairly quickly. Google the English sub version for an interesting comparison.
3.) What age group is Hetalia targeting?
Time for an annoyingly vague answer. While this show is not particularly serious, it is also not intended for 5 year olds. Sexual innuendo is very definitely present, and my best guess for a show rating would be something like PG13, though I can't say for certain. (Though, in actuality, if you were to show this series to a 5 year old the innuendo would probably go right over his/her head. It's not always particularly overt.)
4.) Do I have to be a history buff to enjoy/understand this?
Definitely not. Though I personally love history, this show is largely character driven with only a very light dusting of plot (ie. historic events).
5.) I'm studying WW1/WW2 in history, will this help me?
While it may help you to differentiate between the different countries, it probably won't give you an accurate understanding of what transpired during the World Wars. That said, if you find history dull, this will almost certainly make it more interesting for you, and thus help you memorize various historical events and the motivations behind them.
This is a wonderful show, that in my mind promotes cultural understanding by not pointing the finger of blame at any one country for any of the bloody events of the past, instead choosing to simply make each country lovable in its own quirky way. In my opinion, we need more shows like Hetalia that force us to step back and not take ourselves so seriously for five minutes. In summary, this is easily one of my favorite shows and is in fact one of the only animes I'll watch. It's definitely worth checking out. Five stars.