This is a good place with some genuine highlights though I do think some of the rave reviews of this place are over the top.
Decanter is a very decent steak restaurant with some rather very good quality steaks on offer, and I really enjoyed the bone marrow butter than comes with the steaks (along with a salsa verde, which is nice but nothing special). The starters I thought were distinctly ma-ma. We had the lobster louie and snapper tartare, both very ordinary. The lobster sounded good on paper, served with quail egg, grilled asparagus, fava beans and chipotle yoghurt dressing, but the execution was not very good, resulting in a rubbery and bland lobster. The snapper tartare was boring.
The steaks on the other hand were really delicious, and I might go back for those. Not cheap though, but this is the land of wagyu so the primarily American meat on offer will certainly not be the most expensive beef you have ever seen in Japan.
What the restaurant is most famous for is its wine list, apparently "one of the best in Tokyo". Here it really depends how you define "best". I can completely see why people would feel that way - the wine list is long. Very long, maybe 70 pages, and very democratic as it covers all major regions well. But does it deserve to be called one of the best?
Here you have a restaurant that has the budget to put together a wine list with that many bottles, some of them very rare and expensive (though many are pretty democratically priced actually - for instance a top notch Washington State Cabernet from Woodward Canyon, 2005, is priced at 7,000 yen even though in a shop it would set you back 5,000 yen or more)... and yet there is almost no surprising gem at all. They have played it completely safe. You read though the list, and you find all the usual suspects from all the major regions (this is rare - you tend to find some but certainly not all or most major global wine regions covered well, but Decanter covers most of them very well), but - at least if you are a wine enthusiast looking for this stuff - with very, very few exceptions you don't find yourself exclaiming "wow, that's unusual". Or "what on earth is that??" (I am not talking about bottles worth 100,000 yen or up - I ignore those completely as I can't afford it and very, very few people will even consider those; I am talking about vaguely affordable gems.)
I prefer wine lists that surprise, and many great wine lists in Tokyo do - they will for the most part have fewer wines on the list, but reading such list is not just a boring rattling down of the who-is-who of the various regions.
This is totally a matter of personal taste though - I understand most of Decanter's guests won't be wine fanatics and the fact that they have ample choice from wines from pretty much any significant wine growing region means that Decanter caters for all tastes which is useful when you have a militant new world wine hater at your table in conjunction with a California Cab fanatic who thinks of elegant European wines as "thin", "green" and "boring". I just think it's a shame that some lucky sommelier was blessed with the opportunity to put together a massive wine list for which you need a massive budget, and he/she played it completely safe and boring.
What I do like a lot about the Decanter wine list is the vertical stuff - lots of vintages of the same wine on offer. It would be great to get 10 or so wine enthusiast friends together to do a vertical tasting of one or two different wines.
Another thing I was impressed with was their by-the-glass selection. Not very big, but a bit surprising in places (more so than the wine list). To serve a 2001 Leeuwin Art Series Chardonnay by the glass is incredible. Where do you even get a 2001 these days? Very unusual. I was also impressed with their flexibility. I saw that they offered a Giaconda Chardonnay / Roussane blend which I love as part of a course menu but not by the glass, and asked them if they could possibly make an exception and serve it by the glass given that they did so with a set menu, and they did it without the usual tooth sucking when you ask for something that isn't strictly speaking on the menu (even though I pre-empted my request with a "completely understand if you can't do this", so lazier waiters would have just said 'no' there and then). I would not have minded at all if they had said 'no' - after all, there was no price for it on the menu so fair enough if it's not something they want to do, and I was quite surprised at their flexibility.
I also liked their menu (rather, its format). It is a tablet computer where you can immediately access anything you want (both food and wine). I am normally not at all a gadget man; don't like computers and they don't like me; but this menu I thought was fantastic, very use friendly. The index is very clear and detailed so you can immediately access any type of wine (or food) you are interested in, which makes like a lot easier given the sheer size of the wine list.
So overall a fun experience and I expect to be back. Oh, and they do decent cocktails as well - at least the one I had prior to dinner. I helpfully can't remember what it was called, but it was really good.