Cheap price & very little info as to what cultivar of lilac it is leaves a little to be desired in my opinion. Since this is a plant that you want to have for many years, I guess I'd like to get something that might have so known disease resistance traits. Take a look at the UW-Ext link below, click on the "yellow view PDF" link for some info on lilacs.

Here is a link that might be useful: UW-Ext Lilac article

If the tag just says Syringa Vulgaris without any frou frou type name applied to them, they'll be the common purple lilac you've seen everywhere in the Midwest and the ones people remember from their childhood if they've relocated somewhere where lilacs don't flourish. Despite all the new ones out today, these tried and true lilacs can't be beaten IMHO. But different strokes for different folks.

My neighbor and I both have double lots and have the only two houses on our face of the street. We have a continuous hedge of common lilacs (put in much closer than 20 feet apart) that were planted in 1900. They're show stopping, have never failed to bloom every year and the fragrance is intoxicating. Have never had powdery mildew or any disease; are attended to by an arborist every 10-15 years.

At $3.50 a pop, I hope you picked up a few.

Looks like the lilac hedge of my neighbors that has grown into my yard over many years. The plants "sucker" like crazy and make an impenetrable jungle. I have been selectively pruning out some every year for the past 5 years or so. I have more to do. They get so tall with thick stems, but then they flower very little.