When I think of 'Topshop' I think of Kate Moss who has formerly modeled for and collaborated with Topshop in the past. What do you think of?
I think that this sort of advertising has helped Topshop make a name for itself.
Choice of products and availability

The selection you get to choose from very much depends upon which store you visit. Take Oxford Street for example, you'll find it's massive with so much to choose from- they even sell makeup and you can have your nails done there in store (WAH nails).
Take Kingston, on two levels but still not massive. and take my store in my town, small and not a lot of selection in comparison.
I think this does make a difference on whether or not you'll like the store or not. The more choice you have the more likely you are to come across clothes and accessories that meet your needs. The website is good and easy to navigate and order from, but it's not the same as seeing items in person. They also offer a boutique/ collection on the website (more pricy though).
Types of products offered and more about the store

I find Topshop to be a store where there's often a lot of stuff i would not wear at all, but where I can find nice one offs and some basic things i.e. underwear which I would buy.
The Topshop stores I walk into are usually well set out and stuff is arranged in a way that meets your standards.

Staff and service offered is average and what you'd expect. I've never had anyone come up to me while in store, but the level of service received at till was efficent.
I think the choice of accessories is good but the jewelry can be hit and miss. I've noticed that they've started recently selling earrings similiar to ones worn by Rihanna in the 'We found love' music video, so it's clear they try to keep up with trends and stock items for people interested in taking inspiration in famous people's styles.
Current ranges & my experiences with them

Topshop sells a wide range of clothing but also has a range for tall and petite women. They also stock a maternity wear range.

They sell the same types of clothes you'd expect in any high street chain: dresses, tops, jersey tops, knitwear, jackets, coats, jeans, trousers and leggings, shorts, skirts, suits, playsuits, tights and socks, lingerie, nightwear and swimwear.
They also have a selection of shoes-in my experience the flat ballet shoes are very plain and the colours they offer aren't varied enough. However, I think this depends on the season as they change up their ranges and sometimes you can find nicer stuff in summer for instance.

Their make-up range, based upon me looking at it in stores looks like it'd be average quality. But it's set out very well and the packaging is nice which is what drew me to it in the first place. However, if I were to buy it I would not expect much. It may appeal to teenagers who like experimenting but for women in their 20's it may not be up to your standards.
Summary

Based upon my experiences Topshop is a nice shop where I occasionaly like and buy from, but sometimes walk into and don't like any of the main clothes and find some too boring/ordinary for the price offered. For example a plain top which you could find for a fraction of the price elsewhere. Or some clothes that make me think who the f*** would wear this?
Vouchers?

A tip: If you're on vodafone pay as you go and get 'Vodafone Freebiez' rewards code every time you top up, Topshop vouchers are offered here in exchange for points if you like Topshop and are on a budget.

Keep your eye out for sale items, most of them are as expected but i picked up some nice floral hairbands from there the other day for 1.50!