I've had very poor experiences anchoring into steel studs using toggles. The hole you need to drill is wide, so you're scraping out a lot of the stud just to make it. Especially if you're going into the kind of small, lightweight ~25 gauge steel studs you'll find in most condo apartments, you'll find that putting one of those toggles through a steel stud to be as good or even weaker than simply going through the drywall. They often take 3/8" to 1/2" holes to insert, which is barely lower than the width of a lot of steel studs.
For most applications, use the same thing that is holding those heavy gypsum boards up in the first place: fine-threaded drywall screws. You'll have an easier time going in with the self-drilling kind (they look like they have a little drill bit on the end), though some people go through without them just fine. Personally I like the non-drilling ones and put them through a small pilot hole so I can make sure they don't go in crooked. You can drill 3-4 of those things in in less time than a toggler takes to install. If you need more strength, add more screws. Best part is: this is how your drywall's already being held to the studs, so if you ever remove what you've mounted, you simply drill the screw in a bit below the surface of the wall, plaster over it, and you're done.
By all means, if you have parts of something heavy you're mounting that don't go over the stud, use a toggler behind the drywall there. They're great for that. They'll still take way longer than going into the metal studs though, which takes very little time. With practice you should be able to drill into a stud in ~5 seconds. Toggler's going to take a few minutes at least to put in.
Most drywall screws are only available in the #6-#8 size (and overwhelmingly it's only #6s with a few #7s), so if you need to do something heavy like a bookshelf, wall-mounted desk, mirror, etc., you can get #12 sheet metal screws. They'll take a bit longer to get in, but they're quite strong.
Update: I'm not endorsing any particular brand, but the data sheet here is quite good at listing the strength of sheet metal screws and fine threaded drywall screws in metal. Ctrl-f through for "pull-out" to see all the ratings. Everything in 25 gauge steel (the cheap little studs in most condos) is lab-rated at a minimum of 100lbs tension, and 250lbs shear, and that's for the dinky little #6s. Obviously the safety-rated strength is going to be lower than the lab-failure conditions (it's usually 1/4 the failure weight -- check the recommendations in your area), but it shows you just how much you could hold on 4+ of these guys. The big advantage the togglers have is on popout - if the steel bends for some other reason (e.g. something heavy crashes against the wall), one of the screws could pop out a little bit. Dynamic loads are also a case where togglers may be preferred, as constant pushing and pulling of the screw against the hole could fatigue the metal over time. If you have a static load (e.g. a TV that doesn't move), screwing into the studs should hold it well enough (within reason depending on the TV of course). If you have a dynamic load (e.g. a TV on one of those swivel arms), you might think about putting a toggler behind it for at least a couple mount points. That said, if you're serious about a dynamic load, you should probably take the time to cut out the drywall and install a thick plywood backing board to install the TV over.
