Personally i think nanos are a great introduction to the marine side of the hobby.
The reason for this: They require much more effort and monitoring than larger tanks and i think this provides a really good base for those of us who went through our 'tropical upbringing' barely testing the water at all!!!
marine tanks are fickle creatures that can be fine one day and dying the next, but the focussed attention that a nano tank requires has honed my eyes to seeing when things aren't quite right, before it becomes a major issue.
Tips: steer away from buying existing complete setups. Often they come with underlying issues such as algae/nitrates/poorly fish, and when just starting out these are not issues you want to be contending with. I know its cheaper and visually more appealing to buy a tank full of corals and fish straight away, but you will get more satisfaction and understanding by starting from scratch. You'd be surprised how much enjoyment can be had from 'rock watching'!!
I made this error and spent the best part of 6-9 months fighting against algae issues as the previous owner was using tap water and had awful nitrate and phosphate problems that had leached into the rock.
As has been said don't think that buying small means less expensive. This hobby is pricey but you can make it cheaper. Buying second hand is good, but buying cheap brand new equipment tends to be a false economy so steer clear from '£10 skimmers', cheap lights, and cheap salt brands.
Nanos are not tanks for lots of fish, if anything the fewer the better as maintenance will be easier, and watch out for LFSs saying 'oh yeah they don't get big'. Make sure what you are buying stays small, and doesn't require lots of swimming space. Its just not fair on the animals.
There's loads of other tips, but probably one of the best would be to read through some of the journals on here. Most of the problems you're likely to encounter will be covered within those, and if you're unsure just ask. alot of us have been there, done that, and got the wet floor to prove it.