So, the girlfriend wants to get a cat.
I had cats growing up, but never my own.
We're looking at indoor cats, as we live in a city and only have a small yard.
If you have cats - do you find that leaving them during the day is OK? We'd be gone a maximum of 9 hours, but more realistically 6-7 hours on weekdays.
Any tips?
Any warnings?
I'll wade in with an opinion, as we have 2 cats and being a cat owner means I'm naturally highly opinionated

My cats are also indoor cats and have been all their lives.
Firstly, don't take anyone elses opinion as gospel except that of the vets (and then always contemplate that with a little criticism). Particularly avoid taking to heart the opinion of other cat owners. Cat owners always think their opinion is the correct one, they can be the most repugnant of people. It's expected though, they're passionate about their cats and rightfully so. If you want to keep your cats indoors then do so, the vet told me it was fine, so long as you offered your cat lots of stimulus. Our house looks like kids live with us, there are toys galore. We also have two cats, I wouldn't leave one on its own. Get two to keep eachother company, two is much more fun anyway.
Having indoor cats was a decision based on our location. We are near some main roads and the motorway isn't far either. In the last 18 months of living here we have received several flyers through the door for missing cats. My other half found a dead/dying cat on the main road. My other half is also on a local community facebook page and on there are regular posts for missing cats. There was also a spate of people leaving out poison or anti freeze in bowls of water to kill cats, (some people are mindless idiots). If we lived in the middle of nowhere, surrounded by fields, we'd let our cats roam. We don't, so they don't. Look at it this way, if you had a pet bird, would you throw it out the window every day? If you had a pet fish, would you chuck it in the river everyday? Would you let a young child roam in traffic? Ignore other peoples opinion, make your own mind up, allowing your cat to roam around the streets on its own is a culturally acceptable thing which I don't understand. If I let my dog foul all over the street and your garden I could be fined for it. Why not a cat?
So people will tell you it is cruel to keep your cats indoors, which of course it isn't because the vet told me it isn't and what's more the RSPCA would have been called out by now to take my cats away for cruelty. Other cat owner friends of mine have expressed to me it's cruel, yet these same people continue to just feed their cats dry kibble for their lives. Now I consider that to be cruel. Cats don't tend to drink water and by condemning them to a life of dry cat food you are risking them all kinds of liver, kidney and deficiencies as they grow old. Research long and hard into what you'll feed them.
Be sure you can afford to maintain them. They need neutering, immunisations, regular flea and worm treatment and a monthly insurance plan (if you cannot afford any treatment or surgery for illness and disease yourself).
As for the comments about smell. A cats stool smell is largely dictated by its food intake. Kibble and cheap tinned meat (whiskers and the usual crap) will naturally make your cat poop stink. You can counter this by either feeding them a better diet or using a decent cat litter box and litter. We opted to do both. Our cats seldom smell the house out.
As for the mess they make, well that is something you'll have to accept. It's like having kids, you're going to have to clean up after them. Your home will be as messy as you allow it to become.
We absolutely love our cats, they are the most pampered spoilt little buggers in the world. They are very attached to us and us to them. They generally follow us around the house. Poppy will come sit at the side of my bath and chirp at me, she plays fetch like a dog and is always sat beside you wherever you are. Barry will come in to wake me up every morning and have a cuddle and when he isn't getting attention when we're watching TV he will come play fight with me. Neither of them have ever scratched us, they've been man handled since kittens.
TL: DR
Headlines:
Keep cats indoors if you want
Feed them decent food
Use decent cat litter
Provide lots of toys
Be prepared to spend money
GET TWO!