Before I start, this is my opinion if they're good or not, and not fact.
Wooden Coasters:
To be fair, in my opinion, there's little wooden manufactures can do wrong- they all have their merits.
I'm not going to cover some of the old designers, as before the 80's, most were made by designers and the parks, rather than going to manufactures.
Custom Coasters International: Some very good, some notoriously rough. These were really the first manufacture of modern wooden coasters. Some coasters (such as Shivering Timbers and GhostRider) were mainly out-and-back, and some were more twisted layouts (such as Tonnerre de Zeus). An example would be Megafobia at Oakwood Theme Park:
http://rcdb.com/756.htm
Both these formed just before or after CCI dissolved:
Gravity Group: Good, although can be fairly rough in some coasters. Feature 90 degree banking on some turns, which were unheard of until about seven years ago. Some also have hybrid steel supports, which are unique to wooden coasters. These coasters tend to be some of the biggest modern wooden coasters. An example would be Voyage at Holiday World:
http://rcdb.com/3231.htm
Great Coaster International (GCI): Good. These layouts are generally super compact twister layouts- unlike Gravity Group's huge hills and often out-and-back layouts. The are based on the style of old compact coasters, such as Cyclone at Coney Island. Also, the coasters are generally more family friendly than Gravity Group. An example would be Wodan at Europa Park:
http://rcdb.com/10018.htm
Also, there are a couple of other smaller companies:
Intamin: Good. Modern models use prefabricated track (ie. track is made in a factory, like a steel coaster). These tend to be very smooth, have huge amounts of air-time and are often very tall. An example would be El Toro at Six Flags Great Adventure:
http://rcdb.com/3183.htm
Rocky Mountain Company: Insane. Another user of prefabricated track, but RMC are just ridiculous with it. 80 degree drops, inversions and stupidly banked turns are the order of the day. They also retrace a few coasters that have lived their day. An example would be Outlaw Run at Silver Dollar City:
http://rcdb.com/10582.htm
Steel:
Now here there is a much more varied range of quality:
Arrow: Mainly bad, a few exceptions. Created the first steel coaster (Matterhorn Bobsleds), and then a million looping models, such as Viper at Six Flags Magic Mountain:
http://rcdb.com/25.htm
Then came the boring hyper coasters, such as the Big One:
http://rcdb.com/775.htm
More interesting are the suspended coasters, which is probably their best model. An (insane) example is Eagle's Fortress at Everland:
http://rcdb.com/1337.htm
Also they are notable for introducing the 4th Dimension coasters.
Vekoma: The majority are awful, some are good. They begun ripping off Arrow's Corkscrew model (with one being Alton Tower's very own Corkscrew) before introducing the Boomerang and the that'll you find at almost every crap theme park. Then came the God awful SLC (such as Infusion at Blackpool Pleasure Beach). All these models are very rough and poorly designed.
However, Vekoma have seen the light, and have changed their track style completely. This has brought some magnificent new mine trains (such as Expedition Everest) and inverted coasters (such as Orkanen:
http://rcdb.com/10902.htm). It seems that Vekoma has a better future.
Bolliger and Mabillard (B&M): Good, as in smooth, but some are forceless. Usually at the forefront of roller coaster technology, bringing out the Stand-up coaster, Inverted coaster (Nemesis), Dive Machine (Oblivion), Floorless coaster, Flying coaster (Air) and Wing Coaster. They also made spin offs of classic looping coasters and hyper coasters. Many coasters are very good, but some have been labelled generic and forceless (Nemesis Inferno is an example). The coasters are always well made, though.
Giovanola: Bad. Apart from a few examples (the hyper coasters of Goliath and Titan), Giovanola are pretty bad. Their track is very similar to that of B&M. Many of the other coasters they made aren't too notable.
Intamin: Good. They started off small with a couple of wooden coasters and a few family coasters. However, they got much bigger as the begun creating hyper coasters with huge air-time as well as looping models (Colossus), inverted coasters and launch coasters (Rita). Their coasters are usually fast and thrilling, although not when their trimmed to death (Thirteen).
Schwarzkopf: Good. Creator of the first vertical loop. Most well known for the Looping Star model, and its variants:
http://rcdb.com/r.htm?ot=2&mo=8211, and the Jet Star, and its variants:
http://rcdb.com/r.htm?ot=2&mo=8198. Also producer of Black Hole.
Maurer Söhne: Quite Bad. Horrible reliability and throughputs. They have the spinning coaster (such as Sonic Spinball) and the looping coaster (such as G-Force). They generally have poor restraints.
Gerstlauer: Unreliable, some are good. The Eurofighters are generally good, with some notable (Saw) models with poor smoothness. The bobsled model looks good, and an excellent family coaster. The newest model is the Infinity coaster (such as The Smiler). Lets avoid comment on the reliability of that.
Pinfari: Awful. Nothing more to say, they are awful. Don't try and look for them.
Mack: Good. An up and coming star. Well in place with the powered coaster (Runaway Mine Train) and the bobsled coaster (Avalanche), as well as many other coaster styles that you can see at Europa Park. Then came the mega coaster (Blue Fire), which is said to be Mack's best coaster style. Mack's coasters are slowly being appreciated for being well made and comfortable, and are slowly opening in many parks around the world.
S&S: Even the one's that did open on time are bad. Producer of the gimmicky over 90 degree coaster (such as Mumbo Jumbo), poor reliability launching coasters (Ring Racer) and the hilarious Screamin Squirrel coaster. However, they have shown promise in continuing the 4th Dimension coaster from Arrow.
Morgan: Bad. They make hyper coasters that make Arrow's look good.
Togo. The worst. *Insert slow lift-hill jokes here.*
Premier: Look half-decent. Some promising rides, such as the crazy Poltergeist:
http://rcdb.com/528.htm. Decent trackwork, although there are some dodgy areas.
Have I missed anyone?