Depends where you are -I live in an area with 3 Primarks close by - 1 is huge (Liverpool) and has everything from womens, home, men, kids etc - BUT its very busy all of the time and everyone else seems to know exactly where they are going and I never do!
The second one is smaller but has just been done up - This one ALWAYS used to be a total mess, but is now less messy but there is more stuff so I find it harder to navigate around, its also not very good for buggies so can be a hassle with DS.
The third, my local one is perfect, not too big not too small, just been done up so quicker queing times, easier to navigate but big enough to get a buggy round, it is never as busy but has much more stock which is better arranged.
My advice is to have a good wander round, grab a basket and maybe try a few bits on (Only because their sizes are a little unfair in some items but not all) - Before you pay have a good look at what you are buying and don't be afraid to put some stuff back, thats how I shop there - Some things look like a real bargain but once you've had a think about it, it just isn't right. I do this because I have never known anyone to take anything back to that shop! I think the general thoughts are "Meh, it was only from Primark!"
You have to be careful, you can get some amazing stuff but you could also walk out with a head to toe illuminous yellow jump suit so approach with caution but enjoy it!