Yes, me. I have spent a lot of time working in HR for charities, including two major ones, one very well-known.
I love it. For me personally, the extra sense of job satisfaction that I get (or got) from the fact that the whole purpose of the organisation I was working for was helping people rather than making profits for owners/shareholders was very important. My view certainly wasn't rose-tinted, especially not after having worked for one for more than 5 minutes , but the overall purpose being a Good Thing still helps balance off an awful lot of frustration.
Some charities are very similar to the public sector in terms of operating, many are not any more. Charities need to be professional organisations now with high quality staff and many reflect that. In my first charity the fundraising 'half' of the organisation was more private sector oriented, with a lot of staff with private sector backgrounds. The service delivery 'half' contained more staff with public sector backgrounds, as it's a medical charity.
Charities can be as different from each other as private sector companies are, so don't assume they are all the same, just because their profits are donated.
Staff work very hard and yes there are frustrations, but personally in terms of working in-house on a full-time permanent basis (which I don't do anymore), when I worked for my first charity I knew I'd found the right sector for me.
Lots of people volunteered especially for my first charity with a view of hand-stroking and 'caring' for the recipients of the service provided. Where volunteers were actually needed and used was in fundraising. Marketing experience is a good start, as fundraising is essentially marketing, but if you're having trouble getting in (have you asked for feedback?) some volunteer fundraising experience might be a good start.
If you can pick a specific charity you are very interested in, do. You will probably not have that luxury and actually I don't think that's necessarily a bad thing, although narrowing it down would be good, to a certain type of service provision.
If you're going to commit to volunteering, a charity you have a personal interest in, from a personal experience perhaps, would be the obvious place to go. Something to bear in mind though is that if it's a charity you have a very personal connection with, your view is more likely to be rose-tinted at the moment, therefore your sense of disillusionment when all is probably not rosy will be more. There was permanently a sense of volunteers being frustrated that all the money raised wasn't being spent on services and not necessarily understanding that to get that money, a professional organisation was needed, which costs money. I remember a fundraising manager I worked a lot with explaining to me that she had to put up with a barrage of negativity from volunteers about the fact that she had a car as part of her remuneration. It would have been impossible to do her job without a car, and yet a lot of the volunteers thought fundraising staff should have the same commitment as they did, and (for example) use their own car and get paid very low salaries.
Personally, I had no personal connection with either of the major charities I worked for, although believed they were doing a good and important thing both times. I do have one charity I have a real connection with, that I support financially but have not worked for.
A lot of the fundraising and service provision on the ground for a national charity takes place very locally, and ime working in a region can be very different from working at head office, so be clear about what you want to do.
if you want to ask me anything, do CAT me or email me, email address on my profile.