Usefulness of Metagross and Gardevoir?

I guess Gardevoir can be useful against some dragons in raids if you don't have a team of dragons already. I used to believe Gardevoir was good for gym defense but I've had no trouble punching through them with my full-machamp teams.
I can't find any good use for Metagross anywhere in the meta, which is a shame because I was very excited for it when gen3 was announced.

So, it depends on what you're lacking but unless you are around level 30 or lower and missing on a lot of key legendaries, you won't find a good place for them unless your teams are sub-par at most.

Um.... sorry...

Gardevoir is useful in machamp raids and the other current T3 fighting bosses. She was also useful against double dragon Ray. Metagross can also be useful in machamp raids against Steel moves. I don't think any of them will be useful in coming T5 raids and I wouldn't invest RC in them.

I strongly advise against using rare candies on Pokemon you can hatch/find wild/reasonably walk. Even if they are rare, you can hatch more Beldum and more Ralts. Rare candies are best served to power up legendaries. If you're newer and don't have many of those, Moltres and Zapdos are certainly worthy candidates to power up. If you don't have them yet, or don't want to power them up, I would advise you save the rare candies for future legendaries.

However, to answer your question, I use my Gardevoir a lot against Machamp (and if I can manage to find a Hitmon raid I'll use her against them). Gardevoir with Dazzling Gleam especially shines in cloudy weather, is also an excellent defender (if you're into that sort of thing), and a wonderful gym attacker. I have only managed to hatch a single Beldum, so no Metagross for me. However, what I've read is that Metagross's best role is as a potion-efficient gym attacker. It is capable of soloing Machamp, but so many Pokemon do it better.

TL;DR. Don't use rare candies on anything but legendaries. Gardevoir is better than Metagross.

Just to add to some of the great advice already posted above, there's no particular need to rush into obtaining a powered-up Metagross right now given it's weak moveset and the fact that it's not entirely necessary in the present meta. Best to wait and see if Beldum stars in a future CD and Metagross gets given Meteor Mash.

I've got a 100IV Beldum, which has been evolved into a Metang, and I'm powering it up whenever I get the relevant quests. I've got more than enough candy to max it out and evolve it, but I don't want any of Metagross' current charged moves.

Gardevoir, however, has stellar movesets (esp. DG and SB) to back up strong base stats, and great meta-viability against a number of T3+ raid bosses.

I don't have a good Metagross yet, but I do have 4 Gardevoir, all lv35-40. Three have C/DG, one has C/SB. My SB Gardevoir is primarily used as a gym defender as it's a downgrade to SB Mewtwo, but my DG trio are some of my 3* raid VIPs, and they're amazing gym cleaners as well. Just today they were key in my first Hitmonlee victory, and they have continued to be a regretless investment.

Save your rares for legends, but DEFINITELY get a Gardevoir. It will serve you DANG well, and worst-case you still have a very nice defender with a typing that eats most of the common gym attackers for lunch.

Gardevoir with Conf/DG:
1. Excels in Machamp solos.
2. Great gym defender (walls Machamp and Dragonite).
3. Solid anchor for raiding Dragon-type legendaries.
4. Decent gym attacker (mainly for dealing with Dragon-types).

Metagross with BP/FC:
1. Adds bulk to any Jynx solo team.
2. Great for Granbull solos too.
3. The answer to Gardevoirs in gyms.
4. Good gym defender.
5. Good for raiding the Lati twins.

Metagross with ZH/Pc:
1. Bulky option for soloing (Machamp with Steel moves).
2. Bulky option for soloing Gengar (with Sludge Bomb).

Gardenvoir has one weakness

Since Confusion(Windy) & DG(cloudy) is different type you can't weather boost both at the same time.

I'm waiting to evolve my best metang because metagross has a signature move (meteor mash), which I'm thinking will be introduced during a community event as an evolve-only thing.

However it's worth mentioning that even if the move is awesome, it'd take more fairies and ice types in gyms and raids to make a steel attacker really useful. Rocks are usually better countered by water (hello kyogre), grass, or fighting due to their second type, but he would be the best at countering Lapras, Walrein, and fairies since he resists all their moves and does SE back at them. Still, I'm holding out for a better moveset before investing candies/stardust.

Between the 2, i would definitely choose powering up Gardevoir. Its attack stat, movesets and performance makes it relevant against current gym defenders and in raids.
(Whether or not you use your RC on it is your call, but if have loads of excess RC, then by all means. Otherwise, best kept for other legendaries - Mewtwo, Rayquaza, Raikou, Kyogre, Groudon - 248 candies to max out a single one)

Echoes of what was said before, but here's my take on it:

1) DO NOT use rare candy on Gardevoir, Metagross, (or Tyranitar, Milotic, or even Dragonite). You can walk all of these and get a candy every 5 km. Rare candy is for legendaries, and if you don't do multiple raids a day, every day, only for the top legendaries. It's really difficult to justify powering up an Articuno when you don't have at least 2 Rayquazas maxed out, from a game optimization standpoint. (Of course, this is a game. If it gives you pleasure to power up Articuno, go ahead and do it, but don't expect it to be as useful as a maxed Rayquaza in the game.)

2) Gardevoir is a strong counter for Machamp and Dragons, but only when it's cloudy out. Weather buffed Dazzling Gleam is devastating to things weak to Fairy attacks. That said, there are stronger counters to Latios with Solar Beam or Dragon Claw (and Latias with Thunder). Against Latios with Psychic, Gardevoir shines when it's cloudy (falling just behind Rayquaza in overall effectiveness). Gardevoir is pretty good, but not as good as Rayquaza, Dragonite, or Salamance (all with double dragon moves, of course,) against Lati@s's other charged moves.

3) Metagross is in a similar situation, but it still falls behind all the Dragons against Latios. I suspect if Metagross has steel moves and it was snowing, it would be very good, but there won't be any snow where I live until November at the earliest, so not a priority for me.

Gardevoir has been quite useful against Dragon Legendary's, particularly when they have double-Dragon moves.

Metagross has been an utter disappointment.

As for the Rare Candies... if you have ANY Legendary Pokémon, use the Candies on them instead. Even bad Legendary's. You can walk a Gardevoir or Metagross, but walking a Legendary is almost pointless unless you're walking 100km a day or more.