I've been wargaming for 6-7 years now and came in to possession of a couple of 40k armies so I thought I'd give it a go.  I tried to teach myself the game while playing solo so I could take my time looking up the rules while playing and not feel like I was impacting on someone else's fun.  You can read up on my full experiences here: https://www.beastsofwar.com/project/1359731/

Pros: 

• Of all the games, this is the one you are most likely to find an opponent for, and find for sale in you FLGS
• Very pretty models (Although I'll admit I am not a fan of a lot of them, even if I can't quite work out what it is I don't like)
• Simple mechanics
• Simple gameplay

Cons

• The mechanics might be simple, but there's layers and layers of them and it can get in the way
• I found I needed dozens of printouts to keep track of my games and it made things very confusing
• The games took forever (6+ hours for a 1500 point game, and ITC standard is 2000), but I was learning the rules, and playing both sides.
• Despite having scenario objectives, everything appears to be always decided by who killed the most stuff, and outcome felt like it was mostly dictated by deployment and the first round of play.  This meant I often felt like I was no longer involved in the game and I was little more than a device for rolling dice and applying rules.
• Poor clarity of rules.  A lot of them are open to interpretation, and if you approach them having played other systems they can be real stumbling blocks, even put you off the game
• Due to the way they package their units you can end up buying a a box of 6 models to get just one model, or even one special weapon, and not want the rest.
• The way the unit profiles are laid out is confusing.  Stats in one place.  points costs in another.  Weapon stats in a 3rd location.  Plus, with no art on the profile page I quite often have no idea what the model is they are talking about.  With the space wolves it took me forever to work out which profile was for generic Space marines.  Turns out there were 3 of them, none of them labelled as Space marines

Thoughts

If you just want to put models on the table and have them smash in to each other, and don't care too much about the outcome, and don't mind playing one game in an afternoon, instead of getting a few in, then I can see the fun in this game.  You could have a few drinks, talk some toot, wander from the table occasionally and have a fun casual afternoon.

If you want a balanced competitive game where you are constantly making informed decisions that affect the outcome of the game, then I feel that you will be disappointed.  If you're more in to your narrative play and want to see if your unit of marines can hold the line against the Ork marauders who keep respawning but you have air support coming in 4+1D3 turns if you can last that long, then this could be a fun system to use.

I fear that the game has been around for perhaps too long and is entrenched within it's own systems and legacy.  I would love to see the rules get a full rewrite from the ground up, forgetting all its sacred cows.  It has a lot of potential, but I think it's legacy gets in its own way.  It is a game I want to like, but it's more work, more time consuming, and more cost than the experience deserves, at least for me.  I hear the current edition is the smoothest and best in a long time.  If that is the case, then I hope that the next edition is more for me.  As a 12 year old's first experience, say for 2 brothers to play against each other, I think its good enough as a gateway in and a chance for them to forget the rules and do fun stuff.

I won't go seeking any games of 40K, but if someone turns up looking for a game I'd be willing to give it another try, so long as I don't need to buy more stuff to do so.  Maybe playing against an opponent will show me a different game?