The formation of the alliance will put pressure on Yulia Tymoshenko as the new political bloc could very well secure the most votes at the election and as such will decide who will become prime-minister. Previous polls have shown Yulia Tymoshenko Bloc and a possible Our Ukraine alliance neck and neck for public support. Each with around 16-19% of the vote.
The established alliance may deliver much needed "public relations" benefits as Our Ukraine tries to bolster its public appeal and win back public trust.
The down side is that according to recent polls the People's Self-defence group would receive more then 3% of the vote and as such would have had a reasonable chance of being elected to the parliament in its own right, giving it much more freedom of movement and control over decision making. The upside is that its leaders are guaranteed to be elected in an expanded alliance.
The other possible downside is that whilst a number of people would have been prepared to support and vote for the People's Self-Defence Party as a stand alone party they may not continue to attract the same support as a junior partner in what is seen as a marriage of convenience. On the numbers game People's self-defence would have been better off remaining unencumbered as a stand alone party.
It is early days in the campaign and whilst there will be an expected "honey moon period" as the campaign begins to take form and the issues facing Ukraine are discussed things could go anyway. With around 15% of Ukrainians uncommitted the outcome is still an unknown quantity.
Update (3:32 June 29)
See LEvko's take on Foreign Notes