[One (M7) Q&A, Help & Troubleshooting] xxmorph3u5xx:
1. quirks:

(hardware) camera issues if the phone is manufactured pre-july 2013 (check the forums for details, search for "red tint").

(cosmetic) check for gaps between the white plastic and the aluminium parts of the phone - they are less then 1/5mm but if you're a maniac like me, you will notice, if not you will probably not even notice them.

(hardware) check for dead or stuck pixels - you need to be really thorough as the pixels are sooooo small. The dpi for this phone is ridiculous, you'll love it!

Be sure to get a phone that has valid warranty as all these problems are warranty-covered!

2. sense:

sense is a love or hate thing - i personally love it. it's like a polished aosp with very eye-catching animations and design and much improved features (example: the mail app is brilliant, the calendar is awesome, the browser is blazing fast and still supports flash player, the dialer is brilliant, the camera interface is very well organised, the gallery app is very nice, etc..). If you do not like it you can always go GPE and run aosp roms anyway.

3. storage:

the phone comes in 32 or 64 GB storage variants. For the 32 GB option which i have, the user space available when i do a factory reset is of 29.98 GB.

4. devs:

the devs are a blessing. Really nice and helpful and always keep us up-to-date with custom ROMs

5. root and unlock:

the unlocking process is a piece of cake ans also is rooting. You will find a bunch of very informative tutorials both step-by step text and also video tuts on the forum. If you have difficulties, you can also ask questions on that threads and the users will surely give you a helping hand.

6. s-off:

s-off is basically a more advanced bootloader unlock. It allows you to do more modifications to your phone then bootloader unlock such as: downgrading, flashing custom HBOOTS, flashing radios, etc. The process of S-offing itself is very easy but you have to be very well documented in advance as you have to prepare the phone for the process and s-offing differs according to your current HBOOT version. But, of course, there are very detailed threds regarding this also.

7. beats:

yes, this feature is also available for GPE AOSP

P.S. is you come from the S3 or any other Sammy the first things you will notice:

-the screen will look very unimpressive at first because it's a SLCD and not an AMOLED. This means more accurate color gamut. The AMOLED tends to exaggerate color tones and this makes it look more vibrant. In time though you will notice that the SLCD is superiour in color reproduction as the range of colors it reproduces is higher and also, when you print or look at pictures taken with the phone on a pc screen or tv you will notice that they correspond to what's actually displayed on the phone itself. The dpi capability of the phone's display is also ridiculous..AFAIK it is the best on the market.

-the power/unlock button is idiotically placed (left top side) - WTF were htc thinking..but i got used to it and you will probably do too.

-the build quality and design is superb. The phone feels like a solid piece of matter in your hand. It doesn't squeak or squeal as the sammys plastic casing does. YOU WILL LOVE THIS

-the sound quality is superb. Both on speakers and also on headphones (especially if you have HQ headphones - do not be fooled into buying beats: they are good but too expensive for what they offer. You can get better headphones for that money (i reccomend jays - www.jays.se))

-overall, you will notice that the phone feels more premium, more mature than the samsung. The sammy feels more like a toy, while the htc feels more like a device. Both have their issues and problems but i have come to love this phone and hope you will do too.

Hope i covered all your questions and welcome to the HTC side of Xda!