This is a very timely question, and one that must be discussed fairly and with an open mind.
While we can write long arguments against and for lab grown diamonds, we will try to provide a brief answer to the question.
When talking about a diamond purchase, it's important to try to understand what the buyer has in mind - is it a purchase with deep significance? Or is it a purchase having in mind a fast-fashion easy to wear piece for the everyday person? Or even more important, would this diamond be at all affordable if the buyer would choose a natural option?
As a consequence of these question, we think that an engagement-ring, for example, should always be a natural diamond because it symbolizes the timeless and eternal love of the two lovers. A natural diamond is rare, it keeps value over time and it's a natural product - therefore a perfect fit for an engagement-ring purchase. A lab grown diamond, while it can be as beautiful, it will certainly not appreciate in value over time and is not unique, therefore not a great option for such unique occasion.
However, the value of lab grown can be better seen in extremely rare colors like pink and blue, which have become out of reach of the vast majority of diamond buyers today. Their extreme scarcity make them great investment pieces for high budget investors, but unnatainable for the general market. A lab grown vivid pink or blue is the only option for a budget sensitive buyer, and there is a strong case for this option.
The most delicate question, however, is in regards to melee diamonds used for jewellery. Today's most famous jewellers, like Van Cleef or JAR in Paris, have expensive pieces that are made from small diamonds, and a piece made from small lab-grown diamonds can never command a premium price in the market. For that reason, all leading fashion houses guarantee that their jewels are made exclusively from natural diamonds.
Smaller jewellery brands are tempted to lower prices by adding lab grown small diamonds in their jewellery selection. It's still a little early to know if this will be a step that the major commercial players will take, and for the moment, natural melee diamonds is still the norm in today's jewellery.