I've always found the Welsh, whilst starting with a vague distrust of the English (or other foreigners for that matter), to always end up taking people as they find them regardless of whether they're English or not. I've spent a lot of time in Wales (north and south for holidays, mid wales I've lived in for most of the last 9 months).
Before going to uni in Aberystwyth I was told I'd meet lots of people who'd insult me just for being English, or would totally ignore me. Yet I haven't met a single person like that, even in the infamously Welsh nationalist halls (Pantecelyn) which are for Welsh speakers only. The only times I've seen Welsh people being genuinely rude to the English (as opposed to the usual ribbing during the six nations and football games) have been when the English people in question have been rude themselves - living in somewhere like Aber makes you realise just how bad some English tourists can be.
Some of the people I've seen here make my blood boil - wandering around in England shirts swigging beer, swearing, spitting in the streets, shouting abuse at people wearing Welsh rugby shirts, constantly making comments about sheep shaggers so everybody else can hear them and so forth. Its the minority of English tourists who act in such a manner, but as always its the vocal minority who get noticed and give everybody else from England a bad reputation. Its not as bad or as apparent as on the Costa del Sol granted as amazingtrade alludes to, but its still quite noticeable at times, and it doesn't surprise me that it annoys some of the locals.
Of the other complaints the Welsh make about the English though, the one about the English pushing up property prices seems rather silly though. One of my flatmates lives near Fishguard in Pembrokeshire (South Wales), and she moans that she'll always be unable to buy a house near her current home as all the English moving into the area pushed up the prices. Well sorry, to me thats market forces, which effect everybody and not the Welsh exclusively - for example my cousin can't afford to buy a house in London despite living there all his life. The influx of English people is also reliant on Welsh people who are willing to sell their homes and make as much money as possible in doing so!