I can only tell you about Shadowrun 4e.
The core mechanic calls for 3 kinds of tests:
(1) Success tests. You roll however many d6's you are entitled to, and count up the number of dice you rolled a 5 or 6. These tests are simple and they work alright. They resolve fairly quickly and you can work out your average expected hits very easily.
(2) Extended tests. You roll however many d6's you are entitled to, and you keep rolling them in sets until you get as many successes as you want, as long you are willing to devote more game to resolving the test. These tests are stupid and they are called for all over the place. They take a long time to resolve, and yet the outcome is rarely ever in doubt. There is an optional rule to decrease dice pool size by 1 each time you roll your pool again. This helps a little, but extended test are still pretty stupid.
(3) Opposed tests. 2 opposing characters roll their respective dice pools and the winner subtracts away the losers hits to calculate his net hits. This can be kind of time consuming with large dice pools, especially compared to how fast opposed checks in D20 are resolved, but it not as bad as extended tests. Opposed tests are made frequently in combat.
Character creation:
Shadowrun character creation is VASTLY more complicated than building a level 1 D&D character. Like in D&D, there are tons and tons of trap options.
The biggest trap is that in chargen you buy everything with build points. Anything priced in build points is priced linearly, or almost completely linearly. This means that it doesn't matter if you are going from a 1 rating to a 2 rating in a skill or a 5 rating to a 6 rating. The cost is 4 build points either way.
After character creation, you buy everything in Karma. Karma is stupid. It has scaling costs. Going from a 1 to a 2 rating in a skill costs you four karma. Going from a 5 to a 6 rating costs you 12.
This means you should generally buy all your high numbers in chargen, and then round out your character during the campaign by expending karma. You want to start as a super-specialist, or you will suck forever. Because of this Build Point / Karma system, 2 characters with ALL identical dicepools can be built with enormously different amounts of karma. One guy in our campaign... I rebuilt his character to have all the same dicepools (but having bought high skills and attributes with BP and his low to medium skills and attributes with Karma... the difference was like 93 karma. That is fucking huge. It was like months and months worth of karma wasted.
The worst part of chargen is equipment. A street sam, and most non-magic archetypes are just normal dudes who came into a shit-tonne of cash and bought a bunch of gear and augmentations. They are nothing without their gear, although a fair amount of it is fused to their nervous system, and is thus hard to disarm.
If you like endless lists of gear, all of which require massive customization and modification to be really effective, then I have good news for you. Shadowrun is your new heroin.
The bad news is that it is a tremendous amount of work to generate NPC's who are not speedbumps. Huge amounts of custom equipment and augmentations are generally required to make an effective NPC. So expect a lot of work, unless your PC's are all very bad at their jobs. Then you can just use stock NPC's a fair amount of the time.
Shadowrun provides sample archetypes... the pre-built characters. NEVER use these. I doesn't matter which edition you are playing. Shadowrun is fucking infamous for the terribad quality of it's sample archetypes. For Shadowrun 4e, one prominent poster to the Official Forums compiled his own set of archetypes that are actually pretty dang good. His name was UmaroVI, I believe. His work is stickied somewhere on their forums, last I checked.
Combat:
Rounds last 3 seconds. Characters often take multiple initiative passes in each round, so the in-game clock crawls to a halt. Things like reinforcements and alarms are not really very relevant in combat time because, say that the police/security have a response time of 2 minutes, the players can each take around 120 simple actions within that time frame (assuming 3 IP's for each player... pretty reasonable assumption).
Everyone dies in 2 shots. You have be freaking amazing with firearms to drop anyone you would consider a threat in one hit, but you can punk almost anyone with a double-tap of most weapons if you are merely a competent runner.
Magic is really good. It is so useful that it is a mystery why you would want to do Shadowruns instead of plying your trade legally and making mega bucks for all the super useful stuff you can do. Also, generally it takes magic to fight magic, so team without a magician, is extremely vulnerable to many forms of supernatural assault or harassment. Spirits are completely bad ass. Small spirits are only good for helping you pack or to clean the house. High force spirits are..... have ever seen BMX Bandit VS Angel Summoner?
Vehicles are kinda crazy. They have hardened armor which basically behaves like this: Either you can't scratch it with your weapon, or your split the vehicle open like a .50 cal anti-material round does to a watermelon. There's not a whole lot of in-between.
Matrix
The Matrix rules are complicated an terrible. No 2 GM's ever run them the same way. Most just ignore 80% of the rules and just wing it. As far as I know, every edition of Shadowrun has had terrible Matrix rules, this is by no means a flaw exclusive to 4e. However, I think 4e's wireless matrix is stupid given the lore of the Shadowrun setting, but I understand why the developers wanted to go that direction.
On the Gaming Den forums, there is a huge houserules document called the Ends of the Matrix. It is a coherent compilation of house rules to improve Shadowrun 4e. It was written by one of the authors of 4e. I highly recommend you take a look.
