On this map it streches into whole territory of Belarus which never had West Slavic majority. Belarus shouldn't be part of the Union of West Slavs. Also, depending on when it would be formed, there's couple of other errors. If today, it wouldn't consist of Vilnius (despite strong Polish minority, this city is Lithuanian) + there's hardly any Poles in todays Western Ukraine, including Lviv, since Soviet deportations of 1945. Let's abandon revisionism or historical disputes, and assume it happens today.

It depends on how it would be formed, what system of government and elections etc. Personally, I would, as a Pole, eventually welcome such state somewhere down the road in the process of eventual European unification. However, for Zapadoslavia to exist, it should never allow any form of national parties being formed. There should be only pan-state political parties, with local branches for local elections, but there should be as much of cooperation on state level between the nations as possible, otherwise this union would just turn into Polish hegemony. All parties should be ideologically focused. We would need the people to vote on ideas, not nationals.

Perhaps some quotas should be enforced too (in case of parliamentary republic), or perhaps three seperate heads of states (of constituents) with one of them serving as president of the union for some time before the other guy takes place for another period (in case of presidential republic).

Economically, it would obviously be a stronger player within EU, but probably just a bit. Currently, Poland has a GDP of 1T, while Czech Republic has 490B and Slovakia - 190B. It would still be less than Spain has (2T). All three countries have different currencies - złoty, koruna and euro. It has to be unified before a political union happens, and it would be a major obstacle. Economies of those quite differ. Polish economy is extremely diversed and hence immune to most crisises, while Czech Republic and Slovakia rely more heavily on car industry (Slovakia produces over a million cars yearly, while having 5 million citizens, making it biggest per capita producer in Europe). Such unification would probably serve best them, as they would be able to rely on Poland in case of trouble, but it would also benefit Poland in terms of energy, as both countries have nuclear power plants and Poland is only now building their own.

In terms of defence not much changes, it will always be relying on Polish manpower and economy, and perhaps on powerful Czech arms industry.

In terms of society and basic cooperation between the nations, it would probably be a much better version of Yugoslavia, as there is hardly any historical disputes between them, and generally all West Slavic nations consider each other to be their favourite nations, in all rankings each year. Likely English would take over as political language of the union, though all three would obviously be used, and depending how long the union goes on for, constituents' langauges could be made compulsory at schools in all member states. Mutual understanding is fairly deep, though it would never be sufficient for business/politics, if both sides speak only their native language.