What is the quality of LEGO replicas on Aliexpress?
You might be interested in the excellent, hilarious and comprehensive Communist LEGO report. It tells you all you can expect, from the good brands to the bad brands. At a minimum, it'll get you rolling of your chair with laughter.
A summary taken from the document:

Brand List 2015
LEGO is a global brand and has been the subject of many copies over the years. The US company Tyco were the first with ‘Super Blocks’ in 1984, after LEGO’s brick patent expired in 1983. Almost every attempt at cloning has been inferior, mainly due to production costs being lower resulting in inferior grade materials and poor fitting pieces. Set design has generally varied from mediocre to rubbish. However, the recently invigorated factories of China have begun to churn out various LEGO clones. It’s worth noting that the potential Chinese domestic market is far, far larger than that of Europe and America combined. There are many Chinese companies making LEGO copies, with new ones starting up at an astounding rate. Minifig copying seems to be endemic, taking advantage of enormous demand for collectable box sets that LEGO cannot do due to contractual reasons. A brief trawl of the AliExpress export website finds a list of 50 companies making plastic building blocks, with probably at least half doing LEGO clones. Most if not all are based in Chenghai, the toy capital of China. Some companies appear to source their products from each other and subcontract, so working out exactly who makes which branded kits is not always straightforward. Many are of random quality and there is a lot of chaff and little wheat. The following is an incomplete list of some of the more actively promoted brands. By the time you read this, there will probably be more. It is interesting to note that many Far East websites list the keyword ‘Enlighten’ above ‘LEGO’.

Net: Some knock-off brands are getting really good and compare in quality with LEGO. Some are absolutely horrific. Some are complete cloners (and will likely disappear soon when the LEGO lawyers swoop in), some have found a niche that LEGO doesn't cover and will likely stay around.