A huge amount of Overkill's appeal comes from its novelty and shock value – things that by their very nature are neither sustainable or great for revisiting. On the plus side, it does make the game great for sharing with good-humoured or easily offended friends. This is a game that's great for playing when you've come back from the pub, which is arguably the only time the Wii sees much action from hardcore audiences anyway.
House of the Dead: Overkill may not have a huge amount of longevity in it for single players, but that's not what the series is really about. As long as you can approach the game with that flaw in mind and you aren't going to be massively put off by a bit of swearing and un-PC teasing then Overkill is a great game to buy or rent.