Hey I'm Himeos and i'm a Plat IV League player. I've played some Bard games and tried out different builds and runes and thought I should share my playstyle.
Here's a little edit of Bard I did this week. Play Video

Pros:
- Sustain
- High roaming potential
- Can take champs out of fights for a while (q + r )
- Very fun to play


Cons:
- Unreliable stun
- Ult is hard to use properly
- E is hard to use properly
- No escapes if there is no wall nearby
- Low range except for his ult

Bard's laning phase can be either very good or very bad from my experience so far. You can trade pretty well if u manage to hit a Q on the enemy adc/supp which is quite possible because u can simply hit the minion standing infront of the adc with your Q when he comes to last hit. So you'll want to go for small trades rather than a fight. Your heal (w) supports this kind of playstyle.
Coordination with your jungler is your key to using ur E. Go collect some shimes when you see your adc doing fine and being in no danger and try to start a gank with the Gate where your jungler can simply follow.
Also don't just leave the lane to collect some shimes it's simply not worth letting your adc alone. Try to collect them when your adc either backed and you're still fine or when you just killed the enemies.
Bard struggles vs champions with hard engage such as Leona, thresh etc.
He simply can't do a lot other than stunning both of them with a lot of luck and aim.

In general you'll want to peel for your adc during teamfights. Easier said than done with Bard.
You should try to Q and incoming threats to your adc or apc depending on who does very well and will deal more dmg. Of course you'll want to heal them with ur W which is a nice little movement speed buff. Your Gate probably won't come into use in teamfights but if it does, use it. The enemy will probably follow and that is ur chance for an easy stun using your Q.


The only problem is that Bard's ult is very hard to use properly. In teamfights there's barely a chance to use it that will help anyone. Most of the time it will just turn everyone into a golden statue and nothing is going to happen. What you can do is cancel channels pretty well such as Katarinas etc.. You can also ult if you want one of ur team members to arrive to the fight as it will delay the fight by a few seconds.