The most important thing to do is to only buy from sellers with a good rating that have a lot of feedback and
read the feedback to see if there's any issues with the item or seller.
I haven't had any major problems buying from Aliexpress, but have only bought a few things from there so far.
I bought a 7 inch tablet from a seller - they were quick to reply to any questions I had.
And what amazed me the most was that I received the tablet in only 2 days after ordering it - shipped via DHL to Australia.
About a year later, I got a 5.7 inch Star N9599T phone with 1GB RAM and 8GB ROM from a different seller - there were a few issues with this one, but in the end I'm extremely happy with the purchase. The issues were that the phone was advertised to have an MTK6589T at 1.5GHz, but it only had a 1.2GHz MTK6589 - also the rear camera couldn't focus on anything further than 1m away, so was kind of useless. After contacting, he offered to either send a replacement if I sent it back (not really an option) or he could give me a part refund - since I didn't get the phone as a camera, I didn't mind so much and since the CPU (at 720p) still seemed plenty fast enough for me, I opted for the part refund... but good news was that after rooting the phone and installing the Xposed framework and Gravitybox plugins etc, the camera somehow seemed to fix itself and all of a sudden started taking better photos and video than my old Samsung Galaxy
Also, before ordering, I asked to confirm that he had it in stock so that it would reach me by a certain date (before I left for an overseas trip) and he said he'd send it within 2 days... after 5 days he still hadn't sent it and I was getting worried - he clearly didn't have it in stock afterall. However, just before I was about to cancel the order (8 days), it came up as shipped and 2 days later it was in my hands - so he still kept his promise and it arrived a few days before I left, so I was happy in the end - though a little stressed for a while there
After about 4 months of using this phone now - I'm so happy with it - best phone I've ever had. Everything just works great (after the initial rear camera issue, that somehow fixed itself). Never had any crashes, super stable, stylus is great to use too (though naturally it's not as good as a genuine samsung note pen
but still very useful). OTG works great, bluetooth perfect - only thing missing for me was MHL ... but I knew that before buying it.
On the trip I was using the phone (cruising in the South Pacific), sharing it's wifi net connection via bluetooth with my laptop (Acer), my wife's phone (Galaxy Nexus) and the tablet (bought from the Aliexpress) all at the same time with no problems at all. I didn't even know that was possible! Of course I had another friend with an iPhone that was desperately trying to connect, but being Apple, it just wasn't capable of connecting via bluetooth for net access (I think Apple have them locked down to only streaming audio via bluetooth or something like that??).
Sorry for raving on a bit, but I just wanted to let you know how well the hardware bought from Aliexpress has worked for me.
I also bought some screen protectors on Aliexpress (from yet another different seller). I didn't use DHL for shipping this time, just China Post Air Mail, so it took a while to get here, but I was happy with that transaction as well.
Maybe I've just been lucky, but I will probably buy from Aliexpress again in the near future.
Just make sure you do your research on the device as well as the seller before you buy.
Of course there's always the small chance of getting something from a bad batch - even from the good sellers it can happen occasionally - so there's always a risk, that risk being that it's not worth the cost to send back if you get a dud (depending on where you live) - for the prices I got these things for, it was a risk I was willing to take - but wouldn't recommend it for everyone - YMMV
One other thing, that's probably been mentioned already - the sellers will generally tell you what they think you want to hear, so try to read between the lines
Aliexpress is really just a bit like ebay, but since I have been fairly lucky with the sellers I've dealt with so far, I haven't really had to deal with Aliexpress themselves much at all - so I can't really say for sure how good Aliexpress are when you have a
real problem with a seller.
But like ebay, it's a massive marketplace and really comes down to feedback and ratings - picking an honest seller.
Oh! and one more thing, as a general rule - personally, I would NEVER buy a memory card from Aliexpress or ebay - but that's just me...
Here in Australia, they're cheap enough anyway - and to me, the microSD was very important to be the real thing, as I needed it to be fast since I also use them in a waterproof HD video camera that I take when I go surfing, so also wanted the card to be waterproof and not drop any frames at 1080p 60fps