Are "clip in" pedals and shoes effective for mountain biking?

A) Yes they are worth it.

B) But practice first! A bike shop salesperson basically kidnapped me or held me in the bike shop an extra 20 to 30 minutes. He mounted my bike on an indoor trainer. I did nothing for the next 20 to 30 minutes but practice getting clipped into & OUT of those pedals. Still haven't taken a spill while clipped in. And that was years ago.

Yes they are

I would never recommend shoes with cages and strap off road personally.

The bottom line is if you are going to take MTB seriously you have to choose either flats or clipless. Both systems are good and have bike specific shoes

The flats look up 5 10 shoes and a good pin flat pedal

Clipless requires the shoe with a mounting system and cleat

Cost of either system is close to the same

There is a huge rift with many MTB types that will claim system a or system b is way superior.

there is a learning curve for both

for a beginner I would recommend flats. I have seen many that started out with clipless as well

Go to MTBR.com to get totally confused talk to some locals and see waht they say

it is the roe vs wade of the MTB world

Good luck

I would try pedals with toe clips first. They also make dual platform pedals that are regular on one side and clip in on the other that may be a better choice. The advantage with cycling shoes is the stiffer sole that allows you to apply an even force over your whole foot.

No. Once you get settled, maybe if you want. There is a bit of a learning curve on clipless on trail, and everyone dumps it a few times.

Just use low cut trail shoes and have pedals that accept cage and straps (some don't). The straps don't have to be tight, but they will prevent your foot from slipping off the pedal, something that happens often when starting off going uphill. This often results in a skinned shin.

I have clipless, but I use them on my road bike.

I suggest you NOT start out with cliipless pedals at this point. Especially if you are going to start out riding mtn terrain. You will need your feet loose and available to get off the pedals to put on the ground as fast as is possible.

Later, as your skills improve, you can switch to,cages or cliipless pedals as you choose.

Soccerref

As a downhiller, I don't use them. Most downhillers don't. Just not safe or worth it. As a cross country rider, you MAY want to use them. I would get used to them on road riding first though. About half, maybe more, of cc riders use them. Personally, I wouldn't, I hate them - I ride pretty aggressively/extreme cuts, drags, jumps, slides. Would only use them for road riding.

Yes. But it is personal taste. I have tried flats, but end up with studs clanking my shins too much when off road. Spuds, on the other hand, keep me connected and therefore allow greater control.

Luck