Corporate PAC vs. bundlers
Corporate political-action committees (PACs) are funded by individual contributions of up to $5000 and may make contributions to candidates up to $5000. There may be more restrictive limits in some cases. Corporate PACs are official organizations and are required to report their activities in aggregate and possibly per individual.
Bundlers may collect an unlimited number of individual contributions (up to $2300 per election cycle, where primaries and general elections are separate cycles). Bundlers are not subject to any reporting requirements. Bundlers often bundle in excess of $100,000 per candidate.
Which offers a greater chance for corruption? A $5000 contribution from a corporate PAC that everyone can see which corporation and which candidate? Or a $100,000 contribution from a bundler that comes in the form of fifty individual contributions of $2000? Yet politicians often ban PAC contributions but rarely ban bundlers. Note that bundled contributions may also come from a single corporation.
The benefit of not taking the money through PACs is that she will be able to say that she doesn't take PAC money. It's unclear how valuable this is.
To contrast, in 2016, Open Secrets says that Hillary Clinton received almost $1.8 million from PAC contributions. But she took over $112 million in bundled contributions. Donald Trump took less than $150,000 in PAC money and did not report his bundlers.
I can't tell you how compromising corporate PAC donations are. I can only contrast them with other donations and point out that they are relatively small in size. Amy Klobuchar is only giving up a small amount of contributions relative to her overall potential. It is possible that she will raise enough money from special events where she touts that she doesn't take PAC money to offset the PAC money. We won't know, because that won't be separately reported.
I don't know the correlations between contributions and actions. But this doesn't necessarily mean anything. Was Hillary Clinton pro-choice because Planned Parenthood donated to her? Or did Planned Parenthood donate to her because she was pro-choice? Considering that she has been rather consistently pro-choice since her husband stopped being politically pro-life (as he was in the 1970s), the latter seems more likely to me. The same issue appears with any other correlation.
For example, Joe Manchin may appreciate donations from coal companies. But what he really needs is the votes from coal miners that go with them. He would still support coal mining even if his donations were zero.
Beyond this, since individual donations dwarf PAC contributions, it seems pretty clear that the individual donations, particularly those that come in bundles, include more potential for corruption.