Working at a TK Maxx store can be overwhelming at first if you have no experience in fast fashion retail. There is a lot of manual work, bending and lifting, you can be called to the tills at any time, then return to your department, then help others on other departments, basically there is no idle time for you. Which is good from a business perspective but can be very tiring and some people just can't cope with the demand. You'll do 342854 different things all at the same time.
Sadly, customers will treat you badly. You'll have to put up with non-stop arguments (especially at the tills), I had one where a customer spat at me and others because I refused to give a discount on a £2 pair of socks. This example might be on the extreme end, but be prepared that a lot of customers will think that you are just a vending machine. In the other hand, TK Maxx serves customers on a conveyor belt, which might explain this behaviour from them.
The shifts can be adjusted to your needs if you talk to your manager. Early shifts are the quieter ones and typically offer shorter hours. This is better for people who like to mind their own business as the store isn't very busy, do the job (mainly refilling shelves and rails) and then go home. Late shifts start from around noon and are a lot more busier. You won't do refilling, but maintaining your department (picking up things from the floor and put them back on the shelves) and serving at the tills when required by the manager. Afternoon shifts have an 'open end'. It might say on the rota that you finish at 8 o'clock, but in reality you won't be out of the store before at least an hour after that. Around peak times (Christmas, Easter etc) I regularly did an extra 2-3 hours every day on top of my scheduled times.
This brings us to the salary, which is quite decent compared to other retailers. Early shifts will pay you a reasonable part time income. Late shifts will add more to that and if you work really hard (stay longer when they need you), you can pocket quite some money there. At times I earned £1500 after tax, which is not bad for a retail job. Some colleagues, who volunteer for 'twilight shifts' (very late finish at or after midnight) or even night shifts can take about £2000 home. However, calculating your next paycheck is very confusing due to the funny contracts they offer. Part of your salary is the normal, contracted hours, every month (or 4 weeks, I can't remember), the other part is the overtime you do, BUT this is calculated on a totally different time schedule. You'll get your overtime pay weeks after you actually did the work. Also, these two don't simply overlap, but the overtime schedule changes every month. I have to give them though, that I always got paid in time and fairly, there was no messing around my wages.
You can become a team leader and then deputy manager fairly simply to be honest, as long as you dedicate some time to the job. If you are always available for the extra jobs, work outside your normal hours and just crack on with the things (it won't get any better, so why not just do it without moaning), you can go on a training within the company. Sadly they changed the pay for new team leaders, it's not very appealing anymore.
The main reason I left is because I used to work in the afternoon shifts and it just turned my life/work balance upside down, I had no time to do absolutely anything during the week, but working/sleeping, didn't have a chance to watch a movie in one go. Changing to a lower paid early shift wasn't an option and the new team leader salaries they offer don't justify the amount of work I'd had to put in.
TL;DR: I'd still recommend to work for this company, both the colleagues and managers are a friendly bunch of people to work with, the pay is above the retail average, you can work your backside off if you want and can easily progress on the career ladder.