I've noticed one big difference between the "professional" reviews on major sites and user reviews on Steam/Amazon etc.
By and large, the professional game reviews tend to cluster their scores in the 6/10 - 8/10 range. You have to be exceptionally good to get above that level or exceptionally bad to fall below it. You also - in most cases - get relatively little variation between professional review scores. A game might be 8/10 on one site and 9/10 on another, but it is rare to see a gap larger than 2 or at most 3 points. It does happen - Alien Isolation has had professional reviews ranging from 4/10 to 10/10 - but generally only with unusual games that go outside the usual templates (like Alien Isolation).
User reviews on the other hand, tend to be much more polarised. It's by no means unusual for games to pick up 10/10s from some users and 1/10s for another. Personal biases are much more likely to feature in user reviews ("I'm giving this game a 1/10 because I don't like something the developer said on twitter" or "I'm giving this game a 10/10 because I've spent the last 2 years boring everybody rigid about how good it is going to be and don't want to backtrack"). Often, the scores tend to average out in more or less the same place as the professional reviews once you have enough of both, but with much more divergence on the user reviews.
So which is more useful?
By and large - and with some important caveats - I find the professional reviews more honest and useful. A lot of people complain about the clustering of scores in the 6/10 to 8/10 range, but the nature of the modern games industry (quite risk-averse, with a lot of project oversight) means that most commercially produced games tend to fall into that range. If you assume a 6/10 is "not great, but overall more good than bad" and an 8/10 is "high enjoyable but not ground-breaking", then you're left with a spectrum into which most major releases fit. The industry does throw out the occasional piece of brilliance - which is usually recognised. And sometimes, things go wrong and it throws out the odd turkey (Aliens: Colonial Marines being perhaps the most recent example). When those things happen, most of the big review sites do seem to reflect them.
But those caveats I mentioned before are important. The first is that at the end of the day, the people doing the professional reviews are still human and they still have their own biases, preconceptions and agendas. True, they have people watching them to make sure that they don't give free reign to those... but occasionally, those checks and balances fail. In fact, most of the big review sites have a few known quirks that you learn to watch for. Eurogamer, for instance (which despite the criticism I'm about to hand out, I do, in general, rate highly), has a real Nintendo-nostalgia fetish and a habit of over-scoring first party Nintendo games. At the same time, until fairly recently, it went through a phase of trying to shoehorn political correctness into its reviews and marking down a few games which committed real or perceived transgressions (though I've noticed less of this recently).
The next big caveat with professional reviews is around bugs. The big review sites are often given pre-release copies of games, so that the reviews can go live before release. Indeed, a lack of pre-release reviews is often an early sign that a game will be a turkey (again... Aliens: Colonial Marines had a review embargo until its release day). Thing is, sometimes those review copies are unfinished code. And sometimes they aren't. But regardless, there is a tendancy for professional reviewers to either ignore or to be instructed to ignore bugs, on the basis that "they'll be fixed for release". And, surprise surprise, they often aren't fixed for release. User reviews are often your first warning that a game is a buggy mess - though on PC you do have to try to separate out the inevitable complaints that pop up on every new release's forms to the effect that "it won't run on my 8 year old PC running a pirated and malware-infested copy of the Polish version of Windows 2000".
And the third caveat is, of course, around the risk that professional reviews might have a "bought and paid for" element to them. We all remember the Kane & Lynch scandal from a few years back and there have been other (less prominent) cases since then. Certainly, when you see a positive review for a site which is plastered with full-screen adverts for that game, it's natural to smell a rat.
But of course, in the age of astroturfing, you also have no guarantee that a user-review hasn't been posted by an employee of the developer/publisher or a marketing firm employed on their behalf.