i have been to the similans and surin islands (richelieu rock, koh tachai, koh bon) i think 7 times over the past 3 years. each time i have been there i have seen great great life and reef. for the big stuff i have seen a whale shark, mantas (huge guys about 5m wingspan, they just come up to you look at you and swim around), zebra white tip and black tip sharks (nothing bigger though).
in terms of little stuff the variety of critters is quite remarquable though not as extensive as indonesia.
yes there has been "bang" fishing there. in fact one of the most beautiful sites of the similans, fantasea reef, was completely destroyed. most of this happens off-season (northern hemisphere summer) when there are not tourists and the park rangers just don't look in the right direction.
however i have never been disappointed. richelieu is a fish (and diver) magnet, tachai has brought me amazing interactions with mantas, morning edge (in the similans) has one of the most incredible bommies provinding umpteen opportunities for photos, if you are on a boat that knows as to go to elephant pinnacle (not too far from elephant head but no visible markings) and you will see a fantastic sea fan garden, christmas point will delight you with swimthoughs and zebra sharks. boulder city and shark fin reef will also maybe give you mantas (though not as likely as up in tachai and certainly not as playful). and the similans has a lot more to offer, but i would recommed splitting your time between similans and surin
i have spoken with people who have been to burma and not one of them has been satisfied: destroyed reefs, not sharks, no fish... i have not withnessed it myself so i can only repeat what has been said. this comes froma good friend of mine as well as dive masters in the similans.
plus there is always uncertainty with myianmar: the operators (and there are less and less because of the crap quality) are never certain to be allowed to go there.
the only boat i know that is left doing this burma run is the ocean rover. expensive, luxurious, very very comfortable boat. it also includes similan and surin dives in the trip. but it is expensive, but as far as i know you really get what you pay for so 
you have to be careful selecting your operator in the similans; there are a lot of yahoos and cowboys out there. plus certain boats are more geared towards certain markets: singaporians, japanese, backpackers, etc.... again you get what you pay for...
i have personally been on 2 boats from www.scubacat.com, te scubacat itself and the scuba adventure. the 1st stays in the similans and is a more basic boat, though it is very comfortable and has the biggest dive platform i have seen, check it out. the 2nd one goes up north on 4 day trips. it is much more comfortable, with fewer divers, has air con and a dive leader, pj. i find it to be the better value for money up there, and also scubacat is very security conscious as opposed to many other operator out there ( i was once on a boat that had to return to harbour while we were under water because IT WAS SINKING!!!!!). it is also very flexible when it comes to dive sites. the only down side is that it is not extremely camera friendly (there is a rinse bucket and some outlets but not dedicated photo area)
so is it worth coming all the way from europe and spending so much money? I think so , as i said i have never been disappointed. in fact i just spent 9 days there over xmas and new year and saw mantas at tachai again and again and again...
thailand is a great country with very friendly people and great food. just avoid bangkok and patong (in phuket, stay more towards kata beach or kamala beach).
sorry have to run but please ask more questions, i can try and answer them on the forum or jsut send me an email
/paul