The One X is fantastic, but be careful which reviews you base your buying decision on. There are two distinct versions of the phone: US and international. We (US, obviously) get dual-core 1.5 GHz Snapdragon S4 processors (a very good, great-performing chip). Non US phones get quad-core Tegra 3, which is pretty much top dog among smart phone processors right now.
Benchmarks for the two aren't hugely different, and the S4 actually has some strengths over Tegra 3, but it's still disappointing that the One X we get here is basically just a bigger-screened One S. That said, I own a One S myself and couldn't be happier with its performance.
There are two BIG knocks against the One X. They're the same two problems I have with my One S.
1: NO REMOVABLE FLASH MEMORY. No SD card slot. You're stuck with the 16GB of internal flash space...that's not much.
2: The battery is ALSO not removeable. This is one of my big complaints with the iPhone, and there's no excuse for any phone pulling such stupidity. The One X and One S are both very thin phones (way thinner than an iPhone...my One S looks like you could maybe shave with it) and the non-removable battery contributes to the thinness. But it's not a worthwhile tradeoff. I'd take a couple millimeters in exchange for the ability to swap the battery.
Another thing to consider:
The One X has a Super LCD2 display, while a lot of other high-end phones these days use AMOLED. Both have different strengths (the LCD tends to show better colors but AMOLED has far higher contrast and better blacks) so it's worth deciding whether you have a preference.
Aside from all that, the biggest thing to consider is HTC's Sense overlay. It changes stock Android a lot, and people tend to either love or hate it. I personally hate it. Others don't (usually people who want their phone to be an iPhone, or at least to be shiny like one). If you don't like it, you can install a custom ROM (which you should do anyway if you want to get the most out of your Android phone). But if you're not willing to hack AND you don't like Sense, anything from HTC might not be for you.