SL / Storstockholms Lokaltrafik run all transport in Stockholm from the metro to the buses - that includes everything other than the Arlanda Express which is the best, quickest and most reliable (as well as expensive) way to get to Stockholm-Arlanda Airport from the City (Central Station)
Most of the SL staff I found were pretty friendly however the staff at Underground stations who were working for third-parties on behalf of them, mainly MTR seemed to be really rude and often give me wrong directions which caused me to get stuck on the metro system for close to an hour. (https://www.yelp.co.uk/biz/stockholms-tunnelbana-stockholm)
Unlike most metro stations it is fully accessible - other than certain stations with lift problems (lifts seem to go down all the time with no notice upon leaving a train or on the platform with some metro stations being hard to navigate and even staff unaware that a lift has been down for weeks and possibly even month.
SL, however, do have an accessibility guarantee but with only a Swedish number (expensive to call for foreign tourists) and having to plan in advance - again this can be difficult especially when Underground, I do feel this could be better. That being said, the mobile signal while using the underground network was so much better than others - especially compared to the London Underground which is reliant on Virgin Media wifi at only a few stops.
Transport is pretty good value for money if you buy a pass depending on the length of your stay which could be 24/48/72 hours - if you're a resident or staying longer they do have an SL card which is similar to an Oyster but much worse - you have to pay to get one and then load that card as some transport systems including trams don't have cash facilities - I'd much prefer a system similar to London or Manchester where an Oyster is free (as long as you put money on it) or you can use any contactless card to claim on (and you're still capped paying the cheaper fare) as the system right now is pretty confusing.
My 72-hour card cost 260 SEK which as it included transport all throughout Stockholm County including Taby and Solna on commuter rail, I feel it was value for money as it ended up costing the same as I would have paid for the same period, travelling in Zones 1 and 2 in London (£7 a day)
SL do run many other services too including tram, buses and commuter rail. The metro stations are probably the nicer of the lot, with many key stations having some beautiful art - but others do seem quite dirty with a huge stench of urine and litter which remains unaddressed.
Transport in Stockholm is generally pretty good and SL can be proud of that, with also most trains being level access for disabled people - however, when things go wrong, they seem to go very wrong and with no public email address, making it hard for non-swedes to get in touch with them, the confusing and hard layout of many stations they operate across all networks and the lifts going down more than Mo Salah in the box when he played for Liverpool before VAR was implemented, it does let it down.
3* - Many drawbacks which can make what is generally a very good transport system, like hell. read more