I find the method is different, in English classes they focus on grammar and how to speak in a right way, no matter how much time it might take to you, your classmates can make mistakes all the time but you'll learn with their mistakes and the teacher will correct you all. That's the idea.

Learning with a native, you'll get fluency faster but it doesn't mean you'll speak everything right. The native won't pay attetion in everything you say, just when they don't understand you they'll correct you.

I've been talking only with natives for about six months and that's how it works. Being in English classes helped very much, I'm not that good but my English improved a lot.

It really depends what your purpose is. It is good for maintaining what you have already learned and for developing conversation skills. Also, an advanced learner can help an intermediate learner (though this will mostly benefit the intermediate learner). However, if both of your levels are low, then it will be a frustrating experience because you won't have much to say.

Most of the time it's not very useful. Specially, if you have a better command of English than them. 

I sometimes attend a Spanish conversation group. It's a small group, attendance varies from 4 to 10, and the people who attend vary. When we are lucky we have some native speakers, often we don't. 

I think it is very useful.

Everything helps, everythig is a different kind of learning. Certainly a group of English speakers speaking imperfect Spanish to each other has its problems.

On the other hand, at the end of the evening I have been functioning in Spanish for two hours, and have been expressing and listening to ideas without speaking any words of English.

The fact that the ideas flow naturally--and lead to learning vocabulary words because we actually needed them to say something that we wanted to say--is very helpful.

And after all it's better than spending two hours not speaking Spanish.

It depends. Either both of you, or just one of you, will use Spanish as a crutch when you/they don't know how to say something in English. It can be comforting, like training wheels, and as long as you don't use Spanish too much, it's still practice.

I think on the conversation quality side, speaking with someone from my country is just less interesting in general, but who knows, that person might be still enjoyable to talk with, and a foreigner can still be boring at times.

I guess, give it a go, as long as you diversify your language partners pool, it's just 30/60 minutes a week.