A2 milk refers to milk having a particular type of beta casein. Cows can produce milk with different types of beta casein. These are designated A1, A2, A3, B, C, D and E.
Most European dairy breeds produce milk containing a mixture of A1 and A2 beta casein. Generally speaking the more developed the breed the less A2 beta casein. The concentration of variants other than A1 and A2 tends to be low.
It is relatively easy to select animals that produce A2 milk and to form herds that produce mainly A2 milk.
The company that markets A2 milk, and others, make a range of claims for this milk. The claims are based on a peptide, beta-casomorphin-7-BCM-7, that is produced during digestion of the milk. The claims are based on views that the peptide produced from AI milk (most common type of milk) is harmful but the one produced from A2 milk is not.
The claim that the peptide produced from A1 milk is harmful has been investigated by the independent European Food Safety Agency (EFSA). Scientists studying the claims produced a 107 page report with over 500 references that concluded: "Based on the present review of available scientific literature, a cause-effect relationship between the oral intake of BCM7 or related peptides and aetiology or course of any suggested non-communicable diseases cannot be established." The report* can be downloaded from https://onlinelibrary.wiley.com/doi/10.2903/j.efsa.2009.231r/epdf" rel="nofollow - https://onlinelibrary.wiley.com/doi/10.2903/j.efsa.2009.231r/epdf . There is no cost for this report.
Regarding your query regarding allergy. Smyth et al. (2004)** obtained pure A2 milk and used it for skin-prick testing of 11 milk-allergic children. The tests compared A2 milk with normal (A1/A2) milk. The mean diameter of the wheal raised by normal milk was not significantly different to that raised by A2 milk (8.2 mm for normal milk v 10.7 mm for A2 milk; P = 0.09, paired t test). No patient had a negative reaction to A2 milk when the reaction to normal milk was positive. So no evidence that A2 milk was less allergenic than normal milk.
I am not sure where the allergy claim has come from. The A2 Milk Company do not recommend that individuals allergic to milk drink A2 milk and are clearly aware of the dangers of feeding milk to people who are allergic to it.
A2 milk can be purchased from at least one approved raw milk retailer in England. It is also available pasteurised.
You asked if it was good? The simplest response is that it is too similar to normal milk to be different!
* Scientific Report of EFSA prepared by a DATEX Working Group on the potential health impact of beta-casomorphins and related peptides. EFSA Scientific Report (2009) 231, 1-107.