She brings some utility, but she has about half the DPS output of Tristana for example.
I'd say she's somewhat viable in fast games where no one gets fed or manages to farm up enough money, AND in complete noob games as she's quite straightforward to play.
For anything in between, she's just weak.

Tier lists are made by a bunch of pimple-faced fat guys with nothing better to do.

Ashe is a ranged carry. She hits like a truck if you build her to hit like a truck, and she brings in team fight utility with mass slows and one of the best stuns/initiators in the game in her ult.

Everyone hits like a truck if you build them to hit like a truck. Its just that some others will always hit better.
Example, Tristana has better damage output than Ashe without skills. But then, Tristana also has Rapid Shot that makes her shoot 90% faster. Soooo... If Ashe hits like a truck, Tristana hits like two trucks?

Fixed.

Ashe is generally a low tier champion, and she should be. Ashe is, by far, the easiest champion to learn and master in-game, with the only slight skill required being her skill shot. That being said, she's fine for low to mid tier play, simply because it is so easy to play at high tier level using her, but she falls short at high-top tier as other carries simply bring much more to the table. Odds are you don't play at a high tier level though, in which case you can do well with every champion, so don't worry about it and pick Ashe if you like her.

Ashe is really only a support DPS. She's very lacking in comparison to any carries, as she has no skills that make her a good carry. I've only seen her do good as an assistant, stunning and slowing while pulling a small amount of DPS to help the carries get gold.

Ashe is viable provided that you: kite really well, have exceptional map awareness, can hit 90%+ of your ECA including across the map, can survive mid without dying against a plethora of mostly-better solos (where mid and late game your pacifism turns into control, and control into wins), and that you have a decent supporting cast. She's not quite good enough to be a "primary" carry. She's best suited to a role as Support Carry, IE, she either needs to team up with a Primary Carry or another Support Carry (like Sivir).

Over the past month or so of me deciding to choose Ashe as my main after my long run as Kat, I've proven yet again that a champion that was largely seen as useless and easily countered, with enough skill, can become quite dominant. High/Top Tier is now resembling Mid/Low tier with a varied assortment of players trying out Ashe. We used to never see her, now she's frequently played.

To those thinking she's "lol easy to play easy to master", think again. She's probably the hardest carry to do extremely well with, because she has no escape. Hence, she's almost completely reliant on positioning, kiting, and awareness. Oh, and Plentiful Bounty is useless. One of these days, this ability will be changed, and likely with it Ashe will become an even better champ. So she's one of the few carries that should actually be slated for a buff, rather than a nerf.

She doesn't get much love because the perception since beta has been that she's an easymode champion that falls off after the Mid Elo range. In many cases, these ignorant perceptions are more powerful than reality. While such perceptions should be taken into account, do not overvalue the assumptions of a few. Very few, nay, nobody currently playing the game has a complete understanding of the ins and outs of every champion. It's best that you prove for yourself whether or not something is viable than to go entirely by public opinion on the matter.