Depends what you want the app to do - the majority of 'clients' on the store are apps that connect to an actual torrent client running somewhere else (e.g. On a computer at home). The downloading and uploading doesn't happen on your tablet, it's just a front-end. I have Transmission (torrent client) installed on my router (router has a USB port with a HDD connected) and I can use my tablet with one of those types of apps to control the client wherever I am. The advantage of that is you don't burn your Surface's battery and you can use a stable, wired internet connection for your torrenting.
The disadvantages are that you need a torrent client running somewhere and that the files aren't stored locally on your tablet. Torrex and Torrent RT are both actual, fully fledged torrent clients. All other torrent clients on the store are the type I've described above, so if you want a proper torrent client, the choice is between those two. Torrex is the better of the two as far as I am aware, but both are quite new and sadly often a little unstable. I'm sure they'll be improved over time, though.
If you're interested in the other sort, I'm actually writing an app for remote controlling µTorrent and Transmission as a free app for W8.1 and WP8. There don't seem to be any apps supporting Transmission (there was one but it seems gone now), and frankly every single torrent client on the Windows Store is HIDEOUS, so for my 3rd year uni project I'm writing a torrent client. It'll be out around late January. Ha, that's what I get for not checking the store for a few months. I've been working on and off on mine since September last year but I decided (maybe stupidly) to save it for a uni project. Oh well, let the market decide - if mine's better it'll become popular regardless of competition. That one looks alright but to me it still has the problem a lot do - they take the basic template you get if you make a certain Visual Studio project and then fill it with text.
It's content before chrome which is Metro-y, but it's too much content. Obviously the challenge is finding the balance between too much and too little, and Metro (especially on W8) is still so young that it's hard to follow the patterns.
Functionality is of course king though, as the point of these is you don't have to leave them running so you might not see the UI too much. I've written the entire app (UI excluded, of course) as a library so that I can share it between WP8 and W8 which helps - plus I'll (optionally) sync server settings, etc. Between both, so for those of us with W8 and a WP it'll be great. As for money - I think I'll be putting the app up as an unlimited free trial with a small purchase price as an optional 'donate' button. I've always liked that (ab)use of the buy/try feature. I took ages researching it actually - I wanted a relatively cheap router with support for being a DLNA server and the most recommended router I could see was the. I bought it and was.
Very disappointed. The wifi and internet connection kept dropping, I had to restart it all the time. I thought about returning it but installed custom firmware from and it's made an insane difference. The router is, as said, a DLNA server so I can stream media from it to my Xbox 360, Surface etc., it's a torrent client, ftp server and other nice stuff. I've not restarted it in god-knows how many months, I never have wifi problems or anything.
With the custom firmware it's fantastic, with the stock Asus it's awful. It's running linux so you can telnet in and set up stuff, install custom software and so on.
I'd totally recommend it if you install that firmware.
Downloader Plus RT is an app that makes it both easy and fun to dig through the inexhaustible source of videos that is YT and download the ones you like so you can play them back at will without needing an Internet connection. This modest resource-consuming app not only saves videos from YT on your hard drive at an impressive speed (provided that you have a good internet connection) but also has the ability to extract just the audio from them, which means you can expand your music collection limitlessly. Basically, you hear something you like and you grab it to enjoy it from then on anytime you want as MP3 and not just on your computer. Downloader Plus RT include: - play the movies on external devices like bigger screen TV, computers, gaming consoles with just a tap - upload files on OneDrive - converts clips to mp4 format - extracts audio from videos (as mp3) - totally neat, smart and nice user interface - ability to play videos - SEARCH PLAYLISTS.